- [feature] The Core oeprator system now includes

- [feature] The Core oeprator system now includes
the `getitem` operator, i.e. the bracket
operator in Python.  This is used at first
to provide index and slice behavior to the
Postgresql ARRAY type, and also provides a hook
for end-user definition of custom __getitem__
schemes which can be applied at the type
level as well as within ORM-level custom
operator schemes.

Note that this change has the effect that
descriptor-based __getitem__ schemes used by
the ORM in conjunction with synonym() or other
"descriptor-wrapped" schemes will need
to start using a custom comparator in order
to maintain this behavior.

- [feature] postgresql.ARRAY now supports
indexing and slicing.  The Python [] operator
is available on all SQL expressions that are
of type ARRAY; integer or simple slices can be
passed.  The slices can also be used on the
assignment side in the SET clause of an UPDATE
statement by passing them into Update.values();
see the docs for examples.

- [feature] Added new "array literal" construct
postgresql.array().  Basically a "tuple" that
renders as ARRAY[1,2,3].