Logical toolchains per minor release

[mathstuf]

Describe the problem you are trying to solve

Keeping multiple versions of the toolchain around while updating minor releases easily.

Describe the solution you'd like

I'd like to be able to do rustup toolchain install 1.34 at the release of 1.34.0 and for rustup update to get 1.34.2 after it has released. Right now, I have to keep an eye out for minor updates and manually replace the toolchains I have downloaded.

[kinnison]

This is an interesting idea, though it would nominally require some extra channel data to be held on static.rust-lang.org. We'll certainly consider it.

[mathstuf]

Duplicate of #794.