arch: migrate to "mpack", remove "msgpack"

[vext01]

Hi,

We were discussing in #neovim irc, how neovim uses both mpack and msgpack libraries, and that it could just use one. I don't know the ins and outs of the two libraries, but it would certainly make porting easier to have fewer dependencies.

Thanks.

[justinmk]

Let's try to switch to libmpack for 0.3. cc @tarruda

[tarruda]

Switching neovim to use libmpack was one of my goals when I first started developing it, but I ended up not finding time to focus on this task :(

This will require a bit of refactoring to the RPC modules since libmpack will directly create neovim types. In the end we'll probably have shorter and more efficient code.

I will try to have a look at this later today.

[maisiliym]

There may be some work done on this. Are there any dusty, old branches laying around?

[clason]

Closed by #15566 🎉