networkd: address - add hash helpers for Address objects

networkd: address - add hash helpers for Address objects

Add compare_func and hash_func for the Address object. The notion of
address equality is the same as in the kernel, and hashing preserves
preserves equality.

Two addresses are considered equal if:
   - they have the same address family, and
     - they are neither IPv4 nor IPv6 addresses, or
     - the local addresses are identical, and
       - they are IPv6 addresses, or
         - they have the same prefixlength, and
         - their peer prefixes are identical

This fixes a bug in the old equality check, which got the local address
and the peer prefix mixed up.