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