[Feature Request] Add support for "rustup run active foo"

[lnicola]

rustup can be used to run commands from a specific toolchain, but not from the active one. It would be nice to have a way to do this.

There is a workaround of running the command directly, but there's no guarantee that PATH contains ~/.cargo/bin. Another workaround is to parse the output of rustup show, but it's awkward.

See also rust-lang/vscode-rust#179.

[lnicola]

Closing, see #1279 for a rationale.