The generic hashtable implementation adds a key-value container, that
keeps the key and value inside the hashtable structure and manages
their memory by itself. This data structure is best suited for
fixed-length keys and values.

One creates a new hash table with ht_create and disposes it with
ht_destroy. ht_create accepts the key and value sizes (in bytes) in
addition to the hashing and comparison functions to use. When adding
keys with ht_add, they will be copied into the hash and a pointer to
the value will be returned: data may be put into this structure (or if
the hash table is to be used as a set, one can just not put anything
in).

The hash table comes also with one generic hashing function plus a
comparison function to facilitate ease of use. It also has a custom
hashing and comparison functions for hashing resource IDs with
HashXID.

Reviewed-by: Rami Ylimäki <rami.ylimaki@vincit.fi>
Signed-off-by: Erkki Seppälä <erkki.seppala@vincit.fi>