:class:`.DateTime`.  As Oracle has no "datetime" type per se,
it instead has only ``DATE``, it is appropriate here that the
``DATE`` type as present in the Oracle dialect be an instance of
:class:`.DateTime`.  This issue doesn't change anything as far as
the behavior of the type, as data conversion is handled by the
DBAPI in any case, however the improved subclass layout will help
the use cases of inspecting types for cross-database compatibility.
Also removed uppercase ``DATETIME`` from the Oracle dialect as this
type isn't functional in that context.  fixes #2987

the indexing.  this is for more natural operation.
- also add cast() to the JSON expression to complement astext. This integrates
the CAST call which will be needed frequently.  Part of [ticket:2687].
- it's a little unclear how more advanced unicode attribute-access is going to go,
some quick attempts at testing yielded strange error messages from psycopg2.
- do other cross linking as mentioned in [ticket:2687].

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>