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/sysctl/sysctl.c

@@ -292,7 +292,7 @@ int main(int argc, char *argv[]) {
 
         umask(0022);
 
-        sysctl_options = hashmap_new(string_hash_func, string_compare_func);
+        sysctl_options = hashmap_new(&string_hash_ops);
         if (!sysctl_options) {
                 r = log_oom();
                 goto finish;