add more exprs to CREATE SCHEMA

This patch adds a few additional commands to the list of expressions
accepted by CREATE SCHEMA: CREATE INDEX, CREATE SEQUENCE, and CREATE
TRIGGER are now supported. The latter two are required by SQL 200x,
while the first is merely useful.

Unless anyone objects, I intend to apply this patch by the end of the
week (I might add a few more commands to the set of allowed
expressions first).

-Neil