Use bitflags!() to replace flag.rs

[andelf]

Rust now provides standard implementation of bitflag : rust-lang/rust#13072

@jcmoyer

[jcmoyer]

I'm glad that there's finally a stdlib alternative! Some things I'm concerned about, though:

1. bitflags! doesn't generate implementations for Shl and Shr. We'd still need to implement those ourselves, which implies that the flag_type! macro would have to stay around for a little longer anyways.
2. Likewise, it doesn't implement Not to negate the contained value.
3. The Flags trait I implemented for flag_type! is there to provide a consistent, easy to remember way of computing the none/all bitsets so you don't need to use the awkward flag typename.
4. There's not an easy way to implement the inverse of empty for bitflags! because of (2). Apparently we could have a second impl $BitFlags and add the all function there.

If we want to keep all the functionality of flag_type! but still use bitflags! we'd still have to wrap the use of bitflags! inside of flag_type! and implement the extra traits for it.

[jcmoyer]

On second thought I think we can drop Shl and Shr since these operations don't make sense for a set. I've submitted a PR to rust to implement Not and the inverse of empty (all). If it lands I'll happily tear out flag.rs since the main use cases will be covered yet again. This will introduce breaking changes since we lose the Flags trait.

[jcmoyer]

I'll start working on this since rust-lang/rust#14009 has landed.