hash.h: move object_id definition from cache.h

hash.h: move object_id definition from cache.h

Our hashmap.h helpfully defines a sha1hash() function. But it cannot
define a similar oidhash() without including all of cache.h, which
itself wants to include hashmap.h! Let's break this circular dependency
by moving the definition to hash.h, along with the remaining RAWSZ
macros, etc. That will put them with the existing git_hash_algo
definition.

One alternative would be to move oidhash() into cache.h, but it's
already quite bloated. We're better off moving things out than in.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>