Fix invalid object key hashing in json_unpack() strict checking mode

Fix invalid object key hashing in json_unpack() strict checking mode

The keys which are stored temporarily to a hashtable to make sure that
all object keys are unpacked, were hashed with the object key hashing
function. It is meant to compute hashes for object_key_t values, and
it works incorrectly for plain strings.

Fixed by introducing suitable functions for hashing and comparing
strings for string-keyed hashtables.