Allow converting to or from a u32?

[sunshowers]

Seems like this crate would be most useful if it allowed converting either to or from a (say, serialized) u32.

#18 already exposes new_with_bits, but it seems like that doesn't do any checking for whether the enum even supports those bits. Might it be worth having checked and unchecked variants?

For converting into a u32, a simple impl From<EnumSet> for u32 should be sufficient.