For some reason, when ARRAY was added to the base it was never linked
to postgresql.ARRAY.   Link the two types and also make base
ARRAY the schema event target so that it supports the same
features as postgresql.ARRAY.

Change-Id: I82fa6c9d2b8c5028dba3a009715f7bc296b2bc0b
Fixes: #3964

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

persistence of JSON values in MySQL as well as basic operator support
of "getitem" and "getpath", making use of the ``JSON_EXTRACT``
function in order to refer to individual paths in a JSON structure.
fixes #3547
- Added a new type to core :class:`.types.JSON`.  This is the
base of the PostgreSQL :class:`.postgresql.JSON` type as well as that
of the new :class:`.mysql.JSON` type, so that a PG/MySQL-agnostic
JSON column may be used.  The type features basic index and path
searching support.
fixes #3619
- reorganization of migration docs etc. to try to refer both to
the fixes to JSON that helps Postgresql while at the same time
indicating these are new features of the new base JSON type.
- a rework of the Array/Indexable system some more, moving things
that are specific to Array out of Indexable.
- new operators for JSON indexing added to core so that these can
be compiled by the PG and MySQL dialects individually
- rename sqltypes.Array to sqltypes.ARRAY - as there is no generic
Array implementation, this is an uppercase type for now, consistent
with the new sqltypes.JSON type that is also not a generic implementation.
There may need to be some convention change to handle the case of
datatypes that aren't generic, rely upon DB-native implementations,
but aren't necessarily all named the same thing.

to a :class:`.types.Enum` or :class:`.postgresql.ENUM` subtype
will now emit the expected "CREATE TYPE" and "DROP TYPE" DDL when
the type is used within a "CREATE TABLE" or "DROP TABLE".
fixes #2729

ARRAY type, references #3132

- any/all work for Array as well as subqueries, accepted by MySQL
- Postgresql ARRAY now subclasses Array
- fixes #3516