Implement Rust ABI

[MattWindsor91]

One of the big draws of the phenolphthalein idea is that it'll theoretically support tests written in languages other than C. An obvious way to test this is by implementing a Rust ABI.

I reckon this will require us to:

• Implement a Rust version of Env, Entry, etc;
• Maybe settle any issues wrt how to deal with local variables vs global variables;
• Work out how to compile Rust files against phenolphthalein's API surface (maybe by splitting the project into a library and a binary);
• Implement ABI switching in the runner.