Exclusion constraints on partitioned tables

Hello Hackers,

I'm trying to get things going again on my temporal tables work, and 
here is a small patch to move that forward.

It lets you create exclusion constraints on partitioned tables, similar 
to today's rules for b-tree primary keys & unique constraints:
just as we permit a PK on a partitioned table when the PK's columns are 
a superset of the partition keys, so we could also allow an exclusion 
constraint when its columns are a superset of the partition keys.

This patch also requires the matching constraint columns to use equality 
comparisons (`(foo WITH =)`), so it is really equivalent to the existing 
b-tree rule. Perhaps that is more conservative than necessary, but we 
can't permit an arbitrary operator, since some might require testing 
rows that fall into other partitions. For example `(foo WITH <>)` would 
obviously cause problems.

The exclusion constraint may still include other columns beyond the 
partition keys, and those may use equality operators or something else.

This patch is required to support temporal partitioned tables, because 
temporal tables use exclusion constraints as their primary key.
Essentially they are `(id WITH =, valid_at with &&)`. Since the primary 
key is not a b-tree, partitioning them would be forbidden prior to this 
patch. But now you could partition that table on `id`, and we could 
still correctly validate the temporal PK without requiring rows from 
other partitions.

This patch may be helpful beyond just temporal tables (or for DIY 
temporal tables), so it seems worth submitting it separately.

Yours,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com