ensure correct cast for floats vs. numeric; other fixes

ensure correct cast for floats vs. numeric; other fixes

Fixed regression caused by the fix for :ticket:`9618` where floating point
values would lose precision being inserted in bulk, using either the
psycopg2 or psycopg drivers.

Implemented the :class:`_sqltypes.Double` type for SQL Server, having it
resolve to ``REAL``, or :class:`_mssql.REAL`, at DDL rendering time.

Fixed issue in Oracle dialects where ``Decimal`` returning types such as
:class:`_sqltypes.Numeric` would return floating point values, rather than
``Decimal`` objects, when these columns were used in the
:meth:`_dml.Insert.returning` clause to return INSERTed values.

Fixes: #9701
Change-Id: I8865496a6ccac6d44c19d0ca2a642b63c6172da9