This commit refactors the code to eliminate the use of `using` for
`BigEndian` or `LittleEndian` functions like `HostSwap` and `ReadUint`.
As these functions are frequently used in header files, using the
direct namespace enhances code safety by mitigating potential conflicts
arising from the order of included headers in the absence of explicit
`using` declarations. Generally, avoiding the `using` keyword in headers
is considered a recommended practice.
Additionally, this commit removes the `Encoding` namespace to shorten
the full function names.
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.
Abtin Keshavarzian