Add atomic-traits 0.4 support

[Cannedfood]

This PR adds support for atomic-traits 0.4

There is already a PR by CobaltCause adding support for atomic_traits, but that's for atomic_traits 0.3, and has a bunch of merge conflicts, so I just created a new PR.

[crop2000]

i just came across the same issue.
i chose a different approach.
rather then deriving Atomic Traits here i made this crate compatible to the macros in atomic-traits.
crop2000@a0282e5
so one can simply do:
flier/rust-atomic-traits@f693abc
if either approach is accepted for a pr that would be nice.

[Cannedfood]

That's a also a very clean way of doing this.

Two concerns I'd have with @crop2000's approach:

1. I'd rather not try my luck getting two PRs merged.
2. I'm not convinced that atomic_traits implementing its traits for every atomics library out there is the right approach.

I opened an issue with rust-atomic-traits though asking whether the impl_atomic! macro could be exposed, I think this could allow us to simplify this implementation in a future PR: flier/rust-atomic-traits#10