With NAT64, we need to handle the IPv4 packets inside the border
router functions in OpenThread core.
This commit introduces a few new classes for NAT64:
- Add `Ip4::Cidr` (for specifying CIDR block of translated packets)
- Updated Ip4 address format -- use a union of (m8[4], m16[2], m32)
instead of 4 bytes.
- Extend `Checksum` for supporting ICMP in Ip4 (and TCP4 / UDP4) and
IPv4 header.
Song GUO