Re: Extended Statistics set/restore/clear functions.

>
>
> I agree, and I appreciate you working on it.
>

I've tried to incorporate all your feedback:

* duplicates in pg_ndistinct and pg_dependencies now cause the input
function to fail, with tests for each

* all the non-array, non-parametery stuff moved out of import_mcvlist so
that import_mcvlist can move to mcv.c in a sane way

* whitespace issues

* validating attnums inside pg_ndistinct. I held off on doing
pg_dependencies pending feedback on how I handled pg_ndistinct. tests for
invalid positive (attnum isn't involved in the extended stats object), 0
attnum (just wrong) and invalid negative (more than the number of
expressions in the object)

Still to do:

* attnum checking for dependencies, should be simple, once we've locked
down how to do it for pg_ndistinct

* move import_expressions code to expressions.c along with some other
expression-specific stuff.

* more tests, probably

I'd like to merge these down to 3 patches again, but I'm keeping them
separate for this patchset to isolate the attnum-checking code for this
go-round.