MSI initial support

Tested with WiX 3.9 and msys2 on `i686-pc-windows-gnu` target.
Still need to replace some hardcodes by variables but that's a start.
FYI all this logic of pulling stuff out of the source code will change when we [start dumping the version info into the binary tarballs](https://github.com/rust-lang/rust/pull/20388). I don't think that should affect anything here, besides the necessary rewrite.