Statement-level Triggers For Uniqueness Checks

In digging around the codebase (see thread: Referential Integrity Checks
with Statement-level Triggers), I noticed that unique constraints are
similarly enforced with a per-row trigger.

The situation with unique indexes is fairly similar to the situation with
RI checks: there is some overhead to using a transition table, but that
overhead may be less than the cost of firing a trigger once per row
inserted/updated.

However, there are some significant differences (apologies to everyone
already familiar with this part of the code, it's new to me).

For one, there is no analog to RI_Initial_Check(). Instead the constraint
is initially checked via building/finding the unique index that would
enforce the uniqueness check.

Then, the actual lookup done in unique_key_recheck has to contend with the
intricacies of HOT updates, so I don't know if that can be expressed in an
SPI query. Even if not, I think it should be possible to iterate over
the EphemeralNamedRelation and that would result itself have a payoff in
reduced trigger calls.

I'm going to be working on this as a POC patch separate from the RI work,
hence the separate thread, but there's obviously a lot of overlap.

All advice is appreciated.