Test harness / driver

[nikomatsakis]

We should create a test harness / rustc driver formalityc that works as follows:

• It parses an input rust crate and compiles it to MIR
• It generates MIR formality files from the MIR and some portion of the interfaces of other crates
• It writes those files to a test file and then invokes racket to run MIR formality against them
  • It reads some sort of special comments (e.g., // compile-fail) to determine what behavior is expected.

This can be used to extend our test suite with Rust test files rather than hand-coding things in Racket.

Eventually I expect the racket code to go away and be replaced with either Rust code (like @jackh726's a-mir-formality-rs) or (my current preference) some Rust interpreter of our definition.