Require defining {CRUX,SAW}_RUST_LIBRARY_PATH by default

[RyanGlScott]

Currently, mir-json effectively has two modes of operation:

1. Compile Rust code against the modified version of the Rust standard libraries that ship with crux-mir (here).
2. Compile Rust code against the versions of the Rust standard libraries that come included with rustc.

If the CRUX_RUST_LIBRARY_PATH environment variable (or, equivalently, SAW_RUST_LIBRARY_PATH) is defined when invoking mir-json, then mode (1) is chosen. Otherwise, mode (2) is chosen.

The thing is, mode (2) is very rarely what you want. This is because mir-json is almost always used in tandem with crux-mir or SAW, and as a result, you really want to emit MIR JSON that is compatible with Crucible. The Rust standard library code that comes included with rustc does not meet this requirement for two reasons:

• It does not include the custom tweaks needed to avoid code that Crucible cannot handle.
• It is optimized (unlike the custom standard libraries shipped with crux-mir), which means that inlining functions might expose lower-level functions that Crucible either cannot handle or does not have overrides for.

However, it is far too easy for a mir-json user to forget to define {CRUX,SAW}_RUST_LIBRARY_PATH and accidentally end up in mode (2), which can lead to confusing errors that would not happen under mode (1). (See GaloisInc/crucible#1179 for an example where I fell victim to this confusion myself!)

As a result, I'd like to propose that mir-json should require setting {CRUX,SAW}_RUST_LIBRARY_PATH by default, throwing an error message if they aren't set. It is still possible that a user might want to run crux-mir in mode (2) for debugging purposes, but they should be required to pass an additional flag or environment variable that acknowledges the user knows the risks that they are getting themselves into.