hmap: New functions, macros for iterating buckets without comparing hashes.

hmap: New functions, macros for iterating buckets without comparing hashes.

Usually, the hash comparison that HMAP_FOR_EACH_WITH_HASH does is an
optimization, because comparing a hash value is usually cheaper than
comparing an entire hash map key.  But for simple hash map keys, it makes
sense to just compare the key directly, because it avoids doing two
comparisons when a single simple comparison suffices.  This commit adds new
functions and macros to support this simple case.