Implement automatic Rust source updating.

[kevinpoitra]

This isn't ready for merging yet.

Currently, it downloads the nightly source from Rust's website to the appData/tokamak/ directory, then ungzip and untars it to the /tokamak/rust-sources directory, and makes use of a progress notification.

Stuff left to do:

• [ ] Point Racer to the correct directory for the updated files.
• [ ] Set up configuration for the updater. We'll probably want to support three different groups of users; one who will just let the autoupdater manage everything, another who will want to set the source directory path but let the autoupdater do the rest, and lastly those who would want to manually manage their source files and turn off the auto updater.
• [x] Move file downloading and extracting to a temporary directory and overwrite the actual sources once we've ensured that the extraction was successful.
• [x] Add support for the beta and stable source files.
• [x] Download and store the rustc-xx-src.tar.gz.sha256 checksum for use in update checking and download validation.

[vertexclique]

:+1: @kevinpoitra seems awesome!

[vertexclique]

I will add:

• [ ] Command for updating Rust source

[vertexclique]

bump? @kevinpoitra

[vertexclique]

bump again? @kevinpoitra @PuppyKevin