Support enums in Rust

#### Here's enum support!
The main thing here is test cases in main.rs for enums.
Other than that, there's just a few lines added to the Rust plugin's indexer.py.
I'm also sending over a makefile for test_rust.

And I changed the absolute path to rustc to a lookup from an environment variable RUSTC_DXR, let me know if there's any trouble.