e.g. it never is automatically cast to the type of element in the
structure.  therefore we really don't need the concept of index_map,
it's overkill.   Replace it with a simple callback in Indexable
where each type does its own setup_getitem(), and we're done.
Add a convenience "index_type" attribute to HSTORE and an
"astext_type" attribute to JSON/JSONB..

index_map.   needs generic tests plus migration documentation.
- in particular, the implicit "astext" within cast() is removed.
PG has a built-in cast from json->jsonb and vice versa so we should
stick with requiring explicit if they want astext.cast().

- ARRAY restored, most of JSON, not JSONB yet

without primary keys to be reflected as well as ensured that
a SQL statement involved in foreign key detection is pre-fetched up
front to avoid driver issues upon nested queries.  Fixes here
courtesy Eugene Zapolsky; note that we cannot currently test
Sybase to locally verify these changes.
fixes #3508  fixes #3509

into core expression.   we need to make a sql.elements.Slice object,
which isn't a SQL standard thing; however there's no other place to
put this as long as we want slice handling to have some meaning
in Core.  Basically any other backend that ever wants to use the []
operator, I don't want to re-implement everything for PG on that
backend as well.   this will need to have backend-agnostic tests
in tests/sql/test_operator.py.

to manage re: hstore/json