Add option to read sources from `rustc --print sysroot`

[Boddlnagg]

Standard library sources can now be installed via rustup and are available at the location shown by rustc --print sysroot. We should add an option to the Visual Rust general options page to automatically use this path (and maybe even make it the default).

[Vbif]

I have made some work here. The main problem is: which rustc run to get sysroot? Maybe we need first aggregate ToolVersion and TargetTriple to "toolchain" class. And make toolchain option for project or solution.

[Boddlnagg]

The Cargo wrapper returned by Cargo.FindSupportedInstallation() already has a RustcExecutable property for this purpose. But there already is a GetSysrootAsync method here, which you can probably use. In a later step, we should centralize access to the toolchain, i.e. there should be only a single instance of the Cargo wrapper which handles the currently selected toolchain (related to #242). Toolchain selection will probably be delegated to rustup.

[Vbif]

#251