Add SAW_RUST_LIBRARY_PATH alias for CRUX_RUST_LIBRARY_PATH

[RyanGlScott]

Fixes #43.

[spernsteiner]

I think we should have some kind of prefix on our variable names so there's less risk of name collisions with other tools. This especially applies to the internal / non-user-facing vars ALREADY_SET_PATH, EXPORT_ALL, and USE_OVERRIDE_CRATES. Maybe CRUCIBLE_RUST_LIBRARY_PATH for the one user-facing var (it being the path to the crucible-compatible Rust libraries, and Crucible being the common denominator between Crux and SAW), and CRUX_* for the rest by historical precedent? Or we could allow both CRUX_RUST_LIBRARY_PATH and SAW_RUST_LIBRARY_PATH, so it makes sense for both tools.

[RyanGlScott]

I'd be fine with CRUCIBLE_RUST_LIBRARY_PATH and keeping the rest unchanged. I'll push a new version which does this.

[RyanGlScott]

Hm. Even if we rename it to CRUCIBLE_RUST_LIBRARY_PATH, there is an additional awkward tension in that all of the relevant .rlib/.mir files currently live in crux-mir. I suppose that when we distribute SAW in the future, we will want to include these .rlib/.mir files in the SAW bindist so that they can easily be located.

You know what? I think you've convinced me to pick SAW_RUST_LIBRARY_PATH, as I think that name will be less confusing for people who don't know what Crucible is.

[RyanGlScott]

I've overhauled this PR to (1) introduce SAW_RUST_LIBRARY_PATH as an alias, and (2) revise the documentation to make a clearer distinction between public environment variables like {CRUX,SAW}_RUST_LIBRARY_PATH and other environment variables that are meant to be internal to mir-json-rustc-wrapper.