auto merge of #12533 : alexcrichton/rust/svh, r=brson

These hashes are used to detect changes to upstream crates and generate errors which mention that crates possibly need recompilation.

More details can be found in the respective commit messages below. This change is also accompanied with a much needed refactoring of some of the crate loading code to focus more on crate ids instead of name/version pairs.

Closes #12601