Allow omitting patch version in toolchain specifiers

[kamalmarhubi]

Example use cases:

Installation / update

$ rustup install 1.8
error: toolchain '1.8' is not installed

(I've filed #793 for the confusing error message.)

I think this should install the highest version matching semver requirement ~1.8. For the recent case of 1.12.1, installing 1.12 should therefore install 1.12.1. 1.12.1 should be installed even if the user has 1.12.0 installed.

Run

$ rustup run 1.8 rustc test.rs
error: toolchain '1.8' is not installed

This should be equivalent to specifying the highest installed version matching ~1.8. In this case, I expect it to be equivalent to rustup run 1.8.0 cargo test.

For 1.12, if the user has 1.12.0 installed, then

$ rustup run 1.12 rustc test.rs

should run with 1.12.0, but if they have 1.12.1 installed, it should run with 1.12.1.

Together with #792, this leads to really confusing behavior: rustup run 1.8 cargo test will silently use nightly, since even if you have 1.8.0 installed, that's not 1.8.