Update to rust master
auto merge of #776 : alexcrichton/cargo/issue-488, r=alexcrichton

This helps us not unwrap() and rather propagate the error upwards.

Closes #488
Build lib tests and libraries in parallel

When building unit tests for a library, we don't need the library itself to be
built beforehand, so the two can be built in parallel. If the crate takes awhile
to compile, this shows some excellent wall-time speedups.