Add toolchain management

[emberian]

Currently, users are required to install and manage their own Rust toolchains. Let's make the user's life easier, and do it for them, along the lines of the multirust script for Unix. Feature list:

• Select one of multiple toolchains for use (corresponding to Rust releases/release channels)
• Able to download and install/update toolchains
• Per-project toolchains
• Notifications to update with the ability to auto-update
• Support for both mingw and msvc targets

/cc @brson

[brson]

Yes! I think this would put VisualRust on a path to becoming the way to install Rust on Windows, making it similarly easy to Unix.

[brson]

FWIW I'm intending to switch my OS to Windows and may start finding time to contribute.

[brson]

It would though be very worthwhile to factor the release channel code out into something reusable for other tools.

[emberian]

@brson @alexcrichton what's the status/near-term viability of using only the msvc target?

[emberian]

(my understanding is that it's pretty solid)

[emberian]

Note that I'm explicitly ignoring cross-compiling for now. Can work that out in tandem with the official cross-compiling story.

[emberian]

I'm going to hardcode the PGP key and the HTTPS cert fingerprint, with the ability to override them somehow, to sidestep key management/trust for those.

[brson]

@cmr I believe @alexcrichton is optimistic that 64-bit will be ready to go for 1.3-1.4, but I'm not totally sure.

[alexcrichton]

A non-panicking pretty solid build is ready to go for 1.3, and I hope that we can sort out panicking support by 1.4 for 64-bit at least

[brson]

@Diggsey has a partial port of multirust to rust https://github.com/Diggsey/multirust-rs