Allow use of brillvm as library

[oflatt]

This PR refactors rust brillvm a little to allow use as a library. It depends on pr #319, so the diff includes that work as well.

[Pat-Lafon]

Hmm, this seems a little awkward to me. Are you doing this because you don't want to call brillvm via https://doc.rust-lang.org/std/process/struct.Command.html nor want to re-implement/specialize a version of the main file to your use?(The main part of brillvm, create_module_from_program, is publicly exposed in lib)

[oflatt]

Yeah, we don't want to call brillvm via a process.
I see what you mean- we just need to call create_module_from_program. I'll do that instead.