hashmap: introduce hash_ops to make struct Hashmap smaller

It is redundant to store 'hash' and 'compare' function pointers in
struct Hashmap separately. The functions always comprise a pair.
Store a single pointer to struct hash_ops instead.

systemd keeps hundreds of hashmaps, so this saves a little bit of
memory.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
configure.ac: require automake 1.13

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>