make MsgHdr and MsgHdrMut repr(transparent)

[nerodono]

Closes #633

This PR simply adds #[repr(transparent)] to corresponding types, without any methods for conversion on the types themselves.

[nerodono]

Wrote improvised changelog, hope it's done the right way. By the way, after the merge, can you please release a patch version on crates.io? On my workplace, we decided to use this library for some low-level socket operations, and proposed fix would be required (practically speaking, it's not, since transmuting MsgHdr[Mut] into libc::msghdr won't do anything bad even without repr(transparent), but lack of this guarantee in the API feels wrong) and making git dependency would require to monitor socket2 releases to get rid of it.

[Darksonn]

This is ok with me, but I think we usually do not add changelog entries for unreleased changes.

[nerodono]

This is ok with me, but I think we usually do not add changelog entries for unreleased changes.

So, it's OK to simply not add changelog in this PR?

[Darksonn]

Yes. It will be done in bulk on release.

[Thomasdezeeuw]

You can remove the part for the changelog, otherwise LGTM.

[nerodono]

You can remove the part for the changelog, otherwise LGTM.

Done.

[Thomasdezeeuw]

Thanks @nerodono