Branch channel toolchains on update

It would be nice if there was a way to have rustup automatically branch channel toolchains on update.

What I mean by this is that when I update stable and go from (say) 1.11.0 to 1.12.0, I'd like rustup to automatically rename the current stable-* toolchain to 1.11.0-*, and then install a fresh 1.12.0 compiler in stable-*. This would also be useful for nightlies where I've written something depending on the specific compiler, and want to pin it to a particular version for now.

Currently, I do this by hand by very carefully not running rustup update until I've manually checked to see if there is an update, and if there is, renaming the toolchain directory, and then updating. If I mess up on stable, I need to re-download the entire toolchain that I already had moments before. If I mess up on nightly... well now I need to try and work out what version I had installed in the first place.

Start by tracing the rustup update command and its channel toolchain handling, then review how toolchain directories are renamed or installed. The behavior is complete when updating a channel preserves the previous compiler under its exact version and installs the new compiler under the channel name, including nightly toolchains.