Stability tracking - infrastructure

[nrc]

• docs for managing out of tree tools (cargo, rls, ...)
• rustup - auto-renaming of components (e.g., rls-beta -> rls)
• rustup - nightly only components Not necessary if we are not going to keep tools on nightly
• rustup - dependent components (this would be nice, but doesn't need to block)

[nrc]

cc @brson, @alexcrichton

[nrc]

auto-renaming implemented in rust-lang/rustup#1237

[nrc]

This is done - the last remaining item is optional.

We do need a release of rustup though.