Rewrite (almost) everything

[nuta]

IIRC, this shell is implemented as my first step in Rust long long ago, and it has some faults:

• A monolithic single crate: takes too much time even in an incremental build.
• Async is not used: just because async Rust was still in the alpha stage then.
• Mainloop is fat

This issue aims to deal with these problems by rewriting nsh when I have time (hopefully Q1 2022).

[orhun]

Hey, is there any plans about working on this?