Here are a few procedures that should make using vectors in Elisp a bit
nicer. They are in C since all the Elisp versions I've tried were too
slow for such simple procedures.

I started with just vector-memq, but the other ones should have some
application as well. I took vector-index and vector-partition from SRFI
133 [1].

[1] https://srfi.schemers.org/srfi-133/srfi-133.html