EnumSet lives on in libcollections so that librustc can still use it. This adds a direct dependency on libcollections to librustc and libserialize.
Should not be merged until https://github.com/rust-lang/rfcs/pull/509 is accepted.
All of these collections have already been moved to collect-rs where they will ideally be maintained and experimented with, or will be replaced by something better: https://github.com/Gankro/collect-rs/
[breaking-change]
r? @aturon @alexcrichton