check more features in no-std

[KodrAus]

For #469

This checks a few more features against a proper no-std target, to make sure we're not accidentally pulling in std.

It doesn't currently test v4, because it needs an ambient source of randomness. I think that's possible to do with getrandom, but we also have other methods for constructing a random UUID without needing the v4 feature explicitly.

[KodrAus]

I've replaced our usage of std::sync::atomic with the atomic crate. It looks like it will panic at runtime on platforms that don't support atomic operations. This might not be ideal long-term, but lets us build the v1 feature without needing to complicate our crate features before the has_atomic cfg is stabilized.