Auto merge of #3296 - nrc:check, r=alexcrichton

cargo check

~~This is not finished - the big omission is no tests, and it needs some more testing too. It also requires https://github.com/rust-lang/rust/pull/37681.~~

However, this is my first non-trivial Cargo patch, so I'd like to get some early feedback.

r? @alexcrichton

and cc @rust-lang/tools since this adds a new Cargo sub-command (although we have previously discussed and approved the idea in a tools meeting).

diff --cc src/bin/cargo.rs
Simple merge

diff --cc src/cargo/ops/cargo_compile.rs
Simple merge