Improve rush (the shell in Motor OS)

[lasiotus]

Motor OS has a small unixy shell rush that is incomplete and buggy. It's code is also poorly designed and need to be refactored and cleaned up.

There are several specific issues that need addressing:

• prepended env vars don't work if they contain underscores, i.e. FOO=bar printenv works, but FOO_1=bar printenv does not;
• piping, as in cmd1 | cmd2, doesn't work;
• tabbing support, as in ls foo/, could be useful.

I think using shlex crate, or similar, is warranted. Maybe also glob.

Note: rush shell can be compiled and run on Linux, no need to cross-compile it for Motor OS during development.

[ShubhamK2903]

I can solve this..

[lasiotus]

I can solve this..

Sure, feel free to take a stab! Maybe just post here what specific issue you are working on (fixing env var, piping, etc.), in case other contributors may want to do something similar.