[PATCH] SQL assertions prototype

Various places in the constraint checking code say something like, if we
ever implement assertions, here is where it should go. I've been
fiddling with filling in those gaps for some time now, and the other day
I noticed, hey, this actually kind of works, so here it is. Let's see
whether this architecture is sound.

A constraint trigger performs the actual checking. For the
implementation of the trigger, I've used some SPI hacking for now; that
could probably be refined. The attached patch has documentation, tests,
psql support. Missing pieces are pg_dump support, dependency
management, and permission checking (the latter marked in the code).

This is not a performance feature. It's for things like, this table
should have at most 10 rows, or all the values in this table must be
bigger than all the values in that other table. It's a bit esoteric,
but it comes up again and again.

Let me know what you think.