Conversion functions for IPv4/IPv6 addresses (missing from the NAT64 pr)

This pull request addresses @g-oikonomou comment about missing `ip64_addr_*()` functions in the NAT64 pull request (https://github.com/contiki-os/contiki/pull/804).

These functions are used to convert between IPv4 addresses and IPv4 addresses that are mapped to IPv6 addresses.