Rename "epoch" to "edition"

[klnusbaum]

In rust repo, we've started renaming all the references of "epoch" to "edition". At the request of @Manishearth, let's make any needed parallel changes here in the cargo project.

[Manishearth]

(@klnusbaum is working on this)

[klnusbaum]

I've got a PR ready for this. Just waiting for rust-lang/rust#49035 to land in the nightly

[Manishearth]

Any progress?

[klnusbaum]

I was actually just working on this.

Just put up a PR :)

[matklad]

I believe this is fixed by #5274! 🎉