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.
This commit adds new methods in `Ip6::Header` to get and set Traffic
Class, Flow fields. It also updates and renames some of the existing
methods. This commit also adds a unit test `test_ip6_header` to
validate the IPv6 header parsing and preparation.
Abtin Keshavarzian