Auto merge of #2583 - RalfJung:rustup, r=oli-obk

initial josh subtree sync

This demonstrates what a josh-based rustup would look like with my patched josh. To create it I did
```
git fetch http://localhost:8000/rust-lang/rust.git:start=75dd959a3a40eb5b4574f8d2e23aa6efbeb33573[:prefix=src/tools/miri]:/src/tools/miri.git master
git merge FETCH_HEAD
./rustup-toolchain HEAD && ./miri fmt
git commit -am rustup
```
Unlike the [previous attempt](rust-lang/miri#2554), this does not add a new root commit to the repo.

Once we merge this, we committed to using josh for subtree syncing, and in particular a version of josh that includes josh-project/josh#961 (or something compatible).