Internal refinements to the :class:`.Enum`, :class:`.Interval`, and
:class:`.Boolean` types, which now extend a common mixin
:class:`.Emulated` that indicates a type that provides Python-side
emulation of a DB native type, switching out to the DB native type when a
supporting backend is in use.   The Postgresql :class:`.INTERVAL` type
when used directly will now include the correct type coercion rules for
SQL expressions that also take effect for :class:`.sqltypes.Interval`
(such as adding a date to an interval yields a datetime).

Change-Id: Ifb9f9d7cbd9f5990dcb2abb583193e9e92b789ad
Fixes: #4088

Fixed bug in new SQL comments feature where table and column comment
would not be copied when using :meth:`.Table.tometadata`.

Change-Id: Ib3112e5e02930245daacb36c8ed38c01fa3e7dbd
Fixes: #4087

Added a new method :class:`.DefaultExecutionContext.current_parameters`
which is used within a function-based default value generator in
order to retrieve the current parameters being passed to the statement.
The new function differs from the ``.current_parameters`` attribute in
that it also provides for optional grouping of parameters that
correspond to a multi-valued "insert" construct.  Previously it was not
possible to identify the subset of parameters that were relevant to
the function call.

Change-Id: I6894c7b4a2bce3e83c3ade8af0e5b2f8df37b785
Fixes: #4075

Fixes: #3429

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I870ee7dc801d553c5309c291402ec468b671e9a9
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/383

Drops support for cx_Oracle prior to version 5.x, reworks
numeric and binary support.

Fixes: #4064

Change-Id: Ib9ae9aba430c15cd2a6eeb4e5e3fd8e97b5fe480

Refined the behavior of :meth:`.Operators.op` such that in all cases,
if the :paramref:`.Operators.op.is_comparison` flag is set to True,
the return type of the resulting expression will be
:class:`.Boolean`, and if the flag is False, the return type of the
resulting expression will be the same type as that of the left-hand
expression, which is the typical default behavior of other operators.
Also added a new parameter :paramref:`.Operators.op.return_type` as well
as a helper method :meth:`.Operators.bool_op`.

Change-Id: Ifc8553cd4037d741b84b70a9702cbd530f1a9de0
Fixes: #4063

SQL Server supports what SQLAlchemy calls "native boolean"
with its BIT type, as this type only accepts 0 or 1 and the
DBAPIs return its value as True/False.   So the SQL Server
dialects now enable "native boolean" support, in that a
CHECK constraint is not generated for a :class:`.Boolean`
datatype.  The only difference vs. other native boolean
is that there are no "true" / "false" constants so "1" and
"0" are still rendered here.

Tests are implicit in the existing suites.

Change-Id: I75bbcd549884099fb1a177e68667bf880c40fa7c
Fixes: #4061

Fixed bug in new percent-sign support (e.g. :ticket:`3740`) where a bound
parameter rendered with literal_binds would fail to escape percent-signs
for relevant dialects.  In addition, ensured new table / column comment
support feature also fully makes use of literal-rendered parameters so that
this percent sign support takes place with table / column comment DDL as
well, allowing percent sign support for the mysql / psycopg2 backends that
require escaping of percent signs.

Change-Id: Ia4136a300933e9bc6a01a7b9afd5c7b9a3fee4e3
Fixes: #4054
Fixes: #4052