Rename uthash_memcmp to HASH_KEYCMP, step 3.

This completes the removal of uthash_memcmp,
and the "feature parity" of HASH_KEYCMP with HASH_FUNCTION.

The mental model, again, is: We default to hashing bytes with HASH_JEN,
and we default to comparing bytes with `memcmp` out of <string.h>.
But the user-programmer can override either or both of these defaults
by compiling with `-DHASH_FUNCTION=...` and/or `-DHASH_KEYCMP=...`.