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.

src/shared/fdset.c

@@ -38,7 +38,7 @@
 #define PTR_TO_FD(p) (PTR_TO_INT(p)-1)
 
 FDSet *fdset_new(void) {
-        return MAKE_FDSET(set_new(trivial_hash_func, trivial_compare_func));
+        return MAKE_FDSET(set_new(NULL));
 }
 
 void fdset_free(FDSet *s) {