Remove duplicated `HASH_TO_BKT` logic from all the different hash functions.

This speeds up `HASH_VALUE` by removing a dead assignment, and/or quiets
a compiler warning from compilers that would warn about the dead assignment.
But mostly it just simplifies the code.

Unfortunately, this is an API-breaking change, for any client code that
provides its own `HASH_FOO` function and passes `-DHASH_FUNCTION=HASH_FOO`,
because I'm changing the signature of the `HASH_FUNCTION` function. Fortunately,
at least it's a noisily breaking change; I don't think there's any way
anyone's client code could quietly do the wrong thing after this patch.