Auto merge of #5300 - djc:namespaced-features, r=alexcrichton

Introduction of namespaced features (see #1286)

I think this basically covers all of the plans from #1286, although it still needs a bunch of tests and documentation updates. Submitting this PR to get some early feedback on the direction.

diff --cc src/cargo/core/features.rs
Simple merge

diff --cc tests/testsuite/features.rs
Simple merge