DXR support in Rust

Adds a -Z flag `save-analysis` which runs after the analysis phase of the compiler and saves a bunch of info into a CSV file for the crate. This is designed to work with the DXR code browser, but is frontend-independent, that is this info should be useful for all kinds of code browsers, IDEs, or other tools.

I need to squash commits before landing (there will probably be a fair few to come), please ignore them for now and just comment on the changes.