Alias 'rustup toolchain uninstall' to 'rustup uninstall'

[jordanhiltunen]

See issue: #730.

This PR aliases rustup toolchain uninstall to rustup uninstall, making uninstallation more symmetrical with installation, which already views rustup install and rustup toolchain install as equivalent.

The rustup uninstall alias is directly patterned off of the implementation of
rustup install. I’ve also added a test in cli-rustup.rs to verify.

The issue linked also alludes to some confusing irregularities in the documentation (random flipping between use of 'update' and 'install'), and a softer question about whether rustup toolchain remove should be aliased to rustup remove as well.

This is my first crack at a PR here, so I was a bit sheepish to wade into those more contentious elements of the issue, but I would be glad to dig deeper into the semantic consistency of these various aliases if there's any interest, and someone thinks I'm headed in the right direction. Thanks!

[Diggsey]

@jordanhiltunen Thanks! I don't think we should add further aliases (such as rustup remove) because we want to discourage use of those extra aliases (they only exist for backwards compatibility).

Adding the uninstall alias seems fine to me though because of the symmetry with rustup install, which exists for convenience rather than just backwards compatibility.