Re: operator exclusion constraints

Here's another WIP patch for operator exclusion constraints (both
patches are the same, but one is a context diff made with filterdiff).

It's almost done, and the only reason I'm posting this now is because I
see additional work being done for scalable deferred unique checks, and
I'd like to make sure that we're not interfering with eachother.

Completed Items:

* support for deferred constraints
* enforce constraint when added to a table with existing data
* change predicate to be before DEFERRABLE clause, and require
perentheses around predicate expression
* doc and test updates
* make LIKE and INHERITS ignore operator exclusion constraints (there
was some discussion that we should make LIKE behave more consistently,
but I think that's for a different patch)

Open Items:

* psql support
* pg_dump support
* prevent altering a column that's part of an exclusion constraint in a
way that might cause the exclusion constraint to be violated
* self-review before RRR

Regards,
Jeff Davis