pgsql: Simplify initialization of incremental hash state

Simplify initialization of incremental hash state

The standalone functions fasthash{32,64} use length for two purposes:
how many bytes to hash, and how to perturb the internal seed.

Developers using the incremental interface may not know the length
ahead of time (e.g. for C strings). In this case, it's advised to
pass length to the finalizer, but initialization still needed some
length up front, in the form of a placeholder macro.

Separate the concerns by having the standalone functions perturb the
internal seed themselves from their own length parameter, allowing
to remove "len" from fasthash_init(), as well as the placeholder macro.

Discussion: https://postgr.es/m/CANWCAZbTUk2LOyhsFo33gjLyLAHZ7ucXCi5K9u%3D%2BPtnTShDKtw%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9ed3ee5001b6a2d4cb0166eb8f12a457f30aaca4

Modified Files
--------------
src/backend/catalog/namespace.c | 2 +-
src/include/common/hashfn_unstable.h | 19 ++++++++-----------
2 files changed, 9 insertions(+), 12 deletions(-)