add preliminary support for incremental compilation to rustbuild.py
This implements the integration described in #37929. It requires the use of a local nightly as your bootstrap compiler. The setup is described in `src/bootstrap/README.md`.
This does NOT implement the "copy stage0 libs to stage1" optimization described in #37929, just because that seems orthogonal to me.
In local testing, I do not yet see any incremental re-use when building rustc. I'm not sure why that is, more investigation needed.
(For these reasons, this is not marked as fixing the relevant issue.)
r? @alexcrichton -- I included one random cleanup (`Step::noop()`) that turned out to not be especially relevant. Feel free to tell me you liked it better the old way.
94ae2a2e67