Consider providing the core llvm lib as a Rust crate

[bettinaheim]

We've heard from people using the Rust core of PyQIR directly. I am hence filing this issue for the purpose of discussion and to gather interest in having a Rust package. So far, the Rust API were not necessarily intended to be used directly, and would need to be examined carefully if we were to provide the Rust infrastructure as its own crate.

[cgranade]

It looks like this issue may be relevant for #47 as well, perhaps?

[ss2165]

This may also "solve" #52