Moving ena crate to the rust-lang-nursery


Hello fellow compiler hackers!

Some time ago I created the ena crate, which is just the unification code from rustc extracted. So far, I’ve been moving improvements in ena back into rustc, but now that we can reference crates properly, I want to propose moving ena into the rust-lang-nursery and then removing the relevant code from rustc_data_structures and using ena instead.


Hopefully the beginning of a trend!