Features/move enum set to libstd

[richo]

Move EnumSet to libstd

This moves EnumSet into libstd and updates everything else in core that uses it.

This is the beginning of an attempt to fix#6085

Obviously, this will break any code that attempts to access enum_set as
collections::enum_set, requiring a simple update (What's the mechanism to
communicate that? ping @cwm to put it in This Week in Rust?)

Right now I've left it as multiple commits to make it easier to look over if there are
things I need to fix, I'll squash this if it's ok to merge.

EDIT: This also is somewhat an extension of #12415

[richo]

Thanks to kimundi on IRC, the tests pass locally now.

Please let me know if there's anything more I need to do/consider!

EDIT: And if there's nothing else, I'll squish these all together to avoid polluting history.

[alexcrichton]

Thanks for making progress on #6085! I'd like to discuss this before moving forward, because just using an EnumSet may not be the best solution. For example a bitflags! alternative to EnumSet was proposed in #13072.

This unfortunately suffers from "libstd is what is needed to implement libstd" rather than "libstd is what it should be", because it seems odd to have EnumSet in libstd. Hopefully we can mitigate this before merging!

[richo]

Entirely valid- the decision to move on this came out of a discussion with strcat on irc.

With that said, EnumSet isn't really that much of a collection either, so it seemed somewhat odd in libcollections.

I figure I'm going to keep fiddling with #6085 for the time being, and wait to see what happens upstream with this?

[richo]

Oh, looking closer, bitflags! lives in libcollections too, which would also have the issue of unusability in libstd. Is there an issue for that, and is it something reasonable for someone unfamiliar with rust's core to work on?

[alexcrichton]

Closing due to inactivity. I think the "bit set" data structure is still a bit in flux, and working on #6085 may want to wait for the resolution of this data structure.

[richo]

Sounds good to me. I'd been periodically peeking at this issue and the surrounding ones for movement.