Add windows gui / msi installer [difficult]

To complete the Rust installation experience on Windows we want to be installing rustup via an msi.

Make a proof-of-concept rustup msi installer that embeds libmultirust. The behavior of this msi will be heavily customized - all it does is the standard rustup install, but presented in a windowsy way. To start with it can be really simple:

• On installation, present a screen that says more-or-less what the console installer says now. Have install/cancel buttons.
• After they press install, go to another window that displays whatever status updates we reasonably can as the install is progressing.
• After install show another screen that indicates success.
• Register the uninstaller with windows so it works from the add/remove programs screen.

This will require a lot of refactoring of the existing install code to get it embedded in this new context.

Our options for GUI's will be limited but we can't use something heavy. I'm thinking either something rust-centric like conrod, or just a very thin win32 wrapper.

Start by reading the existing console rustup installer and the libmultirust installation path to understand what must be embedded. Then evaluate a lightweight Rust GUI or thin Win32 wrapper for the install, progress, success, and cancellation screens. Done means a proof-of-concept MSI that performs the standard rustup install and appears in Windows Add/Remove Programs.