Auto merge of #3427 - nrc:api-check-2, r=alexcrichton

Make some aspects of check/build available as an API.

There are two key parts to this commit:
* let API clients run `cargo check` with minimal fuss (ops/cargo_check.rs),
* let API clients intercept and customise Cargo's calls to rustc (all the Executor stuff).

r? @alexcrichton