Introduce `+TOOLCHAIN` syntax for proxies.

[DanielKeep]

If the first argument to a proxy is of the form +TOOLCHAIN, then this
will be interpreted as the toolchain the user wants. That is,

cargo +nightly build

will be equivalent to

rustup run nightly cargo build

This is purely an ergonomic feature designed to reduce typing for people
who frequently switch toolchains (particularly for testing). It is
inspired by a similar feature in the Python launcher.

Why +TOOLCHAIN?

The obvious candidate (used by Python) of -TOOLCHAIN was a no-go.
The issue is that because toolchains can have arbitrary names, it is
difficult to distinguish between a toolchain and an actual argument to
the proxied program in the general case.

@TOOLCHAIN was also considered, as it reads somewhat naturally in
English. However, it turns out that @ is a meta character in
Powershell, which causes the next word to simply disappear from the
command line. Presumably, there is a reason for this.

+TOOLCHAIN was tested and found to not need escaping with CMD,
Powershell, bash, and fish.

[brson]

Cool idea. I'm not sure what I think of the syntax, but your rationale is good. Let's get a build out and see what others think of it!