Speed up collation cache

The blog post here (thank you depesz!):

https://www.depesz.com/2024/06/11/how-much-speed-youre-leaving-at-the-table-if-you-use-default-locale/

showed an interesting result where the builtin provider is not quite as
fast as "C" for queries like:

SELECT * FROM a WHERE t = '...';

The reason is that it's calling varstr_cmp() many times, which does a
lookup in the collation cache for each call. For sorts, it only does a
lookup in the collation cache once, so the effect is not significant.

The reason looking up "C" is faster is because there's a special check
for C_COLLATION_OID, so it doesn't even need to do the hash lookup. If
you create an equivalent collation like:

CREATE COLLATION libc_c(PROVIDER = libc, LOCALE = 'C');

it will perform the same as a collation with the builtin provider.

Attached is a patch to use simplehash.h instead, which speeds things up
enough to make them fairly close (from around 15% slower to around 8%).

The patch is based on the series here:

https://postgr.es/m/f1935bc481438c9d86c2e0ac537b1c110d41a00a.camel@j-davis.com

which does some refactoring in a related area, but I can make them
independent.

We can also consider what to do about those special cases:

* add a special case for PG_C_UTF8?
* instead of a hardwired set of special collation IDs, have a single-
element "last collation ID" to check before doing the hash lookup?
* remove the special cases entirely if we can close the performance
gap enough that it's not important?

(Note: the special case in lc_ctpye_is_c() is currently required for
correctness because hba.c uses C_COLLATION_OID for regexes before the
syscache is initialized. That can be fixed pretty easily a couple
different ways, though.)

--
Jeff Davis
PostgreSQL Contributor Team - AWS