2. compiler names changed to be less verbose, unused classes removed.
3. Methods on Dialect which return compilers, schema generators, identifier preparers
have changed to direct class references, typically on the Dialect class itself
or optionally as attributes on an individual Dialect instance if conditional behavior is needed.
This takes away the need for Dialect subclasses to know how to instantiate these
objects, and also reduces method overhead by one call for each one.
4. as a result of 3., some internal signatures have changed for things like compiler() (now statement_compiler()), preparer(), etc., mostly in that the dialect needs to be passed explicitly as the first argument (since they are just class references now).  The compiler() method on Engine and Connection is now also named statement_compiler(), but as before does not take the dialect as an argument.

5. changed _process_row function on RowProxy to be a class reference, cuts out 50K method calls from insertspeed.py

- added some generative methods to exists()
- got clause adapter to work with join()

- fixed doc typo ("in_" operator)
- misc indent stuff

to select() statements; i.e. eagerloader is better at locating the correct
selectable with which to attach its LEFT OUTER JOIN.
- some fixes to new tests in inheritance5 to work with postgres

uses operator precedence to more intelligently apply parenthesis
to clauses, provides cleaner nesting of clauses (doesnt mutate
clauses placed in other clauses, i.e. no 'parens' flag)
- added 'modifier' keyword, works like func.<foo> except does not
add parenthesis.  e.g. select([modifier.DISTINCT(...)]) etc.

- ClauseVisitor has handy chain() method.

SchemaItem so that the traversal of items is controlled by the
ClauseVisitor itself, using the method visitor.traverse(item).
accept_visitor() methods can still be called directly but will
not do any traversal of child items.  ClauseElement/SchemaItem now
have a configurable get_children() method to return the collection
of child elements for each parent object. This allows the full
traversal of items to be clear and unambiguous (as well as loggable),
with an easy method of limiting a traversal (just pass flags which
are picked up by appropriate get_children() methods). [ticket:501]
- accept_schema_visitor() methods removed, replaced with
get_children(schema_visitor=True)
- various docstring/changelog cleanup/reformatting

requires that the target column be present in a sub-element of the
target selectable.
- embedded logic above more appropriate for ClauseAdapter functionality
since its trying to "pull up" clauses that represent columns within
a larger union up to the level of the union itself.
- the "direction" test against the "foreign_keys" collection apparently
works for an exact "column 'x' is present in the collection", no proxy
relationships needed.  fixes the case of relating a selectable/alias
to one of its underlying tables, probably fixes other scenarios

'duplicate' columns from the resulting column clause that are known to be
equivalent based on the join condition.  this is of great usage when
constructing subqueries of joins which Postgres complains about if
duplicate column names are present.
- added support to polymorphic stuff for more than one "equivalent column",
when trying to target columns in the polymorphic union; this applies
to multi-leveled inheritance
- put above-two concepts together to get the abc_inheritance tests to work
with postgres