Created new visitor system called "internal traversal" that
applies a data driven approach to the concept of a class that
defines its own traversal steps, in contrast to the existing
style of traversal now known as "external traversal" where
the visitor class defines the traversal, i.e. the SQLCompiler.

The internal traversal system now implements get_children(),
_copy_internals(), compare() and _cache_key() for most Core elements.
Core elements with special needs like Select still implement
some of these methods directly however most of these methods
are no longer explicitly implemented.

The data-driven system is also applied to ORM elements that
take part in SQL expressions so that these objects, like mappers,
aliasedclass, query options, etc. can all participate in the
cache key process.

Still not considered is that this approach to defining traversibility
will be used to create some kind of generic introspection system
that works across Core / ORM.  It's also not clear if
real statement caching using the _cache_key() method is feasible,
if it is shown that running _cache_key() is nearly as expensive as
compiling in any case.    Because it is data driven, it is more
straightforward to optimize using inlined code, as is the case now,
as well as potentially using C code to speed it up.

In addition, the caching sytem now accommodates for anonymous
name labels, which is essential so that constructs which have
anonymous labels can be cacheable, that is, their position
within a statement in relation to other anonymous names causes
them to generate an integer counter relative to that construct
which will be the same every time.   Gathering of bound parameters
from any cache key generation is also now required as there is
no use case for a cache key that does not extract bound parameter
values.

Applies-to: #4639
Change-Id: I0660584def8627cad566719ee98d3be045db4b8d

The :class:`.BakedQuery` will not cache a query that was modified by a
:meth:`.QueryEvents.before_compile` event, so that compilation hooks that
may be applying ad-hoc modifications to queries will take effect on each
run.  In particular this is helpful for events that modify queries used in
lazy loading as well as eager loading such as "select in" loading.  In
order to re-enable caching for a query modified by this event, a new
flag ``bake_ok`` is added; see :ref:`baked_with_before_compile` for
details.

A longer term plan to provide a new form of SQL caching should solve this
kind of issue more comprehensively.

Fixes: #4947
Change-Id: I5823c4fa00e7b6d46a2e8461b02d8b16605a6ed0

In order for text(), custom compiled objects, etc. to be usable
by Query(), they are all targeted by object key in the result map.
As we no longer want Query to implicitly label these, as well as that
text() has no label feature, support adding entries to the result
map that have no name, key, or type, only the object itself, and
then ensure that the compiler sets up for positional targeting
when this condition is detected.

Allows for more flexible ORM query usage with custom expressions
and text() while having less special logic in query itself.

Fixes: #4887
Change-Id: Ie073da127d292d43cb132a2b31bc90af88bfe2fd

This performance critical method is on Query needlessly, just to appease
the horizontal sharding API.   Have the performance impact of invoking
Query only incur if horizontal sharding is actually used.

Change-Id: I03db2befe2f5614380258927a62ed389a6ba0fae

Fixes: #4850

<!-- Provide a general summary of your proposed changes in the Title field above -->

### Description
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Removes usage of `util.callable`.

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #4851
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4851
Pull-request-sha: a0ccdff2cb74f5e944d8baccc269c382b591c8e2

Change-Id: I79918f44becbc5dbefdc7ff65128695c1cabed1d

in dfb20f07d8, we merged the removal of the threadlocal strategy
from the 1.4 branch.   We forgot to take the docs out :).

Also, there seems to be an uncovered _contextual_connect call
in horizontal shard.  Cover that and repair.

Fixes: #4632
Closes: #4795
Change-Id: Id05cbbebe34a8f547c9c84369a929a2926c7d093

commit d4f3bedc74568b7ec543988ee2d43e64c5ace28f
Author: Carson Ip <carsonip@users.noreply.github.com>
Date:   Fri Jul 5 11:20:12 2019 +0800

    Fix typo in docstring

commit a3e4b05744f51ec5d12a2fee1ad6093de904273e
Author: Carson Ip <carsonip@users.noreply.github.com>
Date:   Fri Jul 5 11:14:57 2019 +0800

    Fix typo in docstring

Change-Id: Ifa2ebff5629bf970e5fac28bba64d501376cfae9

Fixed bug where the :attr:`.Mapper.all_orm_descriptors` accessor would
return an entry for the :class:`.Mapper` itself under the declarative
``__mapper___`` key, when this is not a descriptor.  The ``.is_attribute``
flag that's present on all :class:`.InspectionAttr` objects is now
consulted, which has also been modified to be ``True`` for an association
proxy, as it was erroneously set to False for this object.

Fixes: #4729
Change-Id: Ia02388cc25d004e32d337140b62a587f3e5a0b7b

Fixed regression where new association proxy system was still not proxying
hybrid attributes when they made use of the ``@hybrid_property.expression``
decorator to return an alternate SQL expression, or when the hybrid
returned an arbitrary :class:`.PropComparator`, at the expression level.
This involved futher generalization of the heuristics used to detect the
type of object being proxied at the level of :class:`.QueryableAttribute`,
to better detect if the descriptor ultimately serves mapped classes or
column expressions.

Fixes: #4690
Change-Id: I5b5300661291c94a23de53bcf92d747701720aa1

A major refactoring of all the functions handle all detection of
Core argument types as well as perform coercions into a new class hierarchy
based on "roles", each of which identify a syntactical location within a
SQL statement.  In contrast to the ClauseElement hierarchy that identifies
"what" each object is syntactically, the SQLRole hierarchy identifies
the "where does it go" of each object syntactically.   From this we define
a consistent type checking and coercion system that establishes well
defined behviors.

This is a breakout of the patch that is reorganizing select()
constructs to no longer be in the FromClause hierarchy.

Also includes a rename of as_scalar() into scalar_subquery(); deprecates
automatic coercion to scalar_subquery().

Partially-fixes: #4617
Change-Id: I26f1e78898693c6b99ef7ea2f4e7dfd0e8e1a1bd