Add Bitfield cut operator and other improvements

[timvermeulen]

This PR adds revamped bitfield combinator logic, which was necessary for the cut operation, and the other combinators (union / intersection / difference) benefit from it as well. Mainly:

• a Combinator trait that lets you concisely specify how two bitfields should be combined
• a Combine range iterator that actually combines them according to the specified Combinator

This logic (+ explanations) can all be found in the combine.rs file. I also threw in the symmetric difference operation (usable with the ^ and ^= operators) because of how easy it was with the new setup.

The benchmarks didn't show a noticeable difference compared to before these changes.

Other changes:

• Removed the BitField::{merge, intersection, difference} methods because they returned range iterators and we didn't use them, the corresponding operators still work like before
• Renamed the only internally used merge method to union in order to avoid confusion with the inernal Merge type in combine.rs that is subtly different
• Created a new bitfield/iter module, so the diff is a bit larger than the actual changes

Closes #602

[austinabell]

Suggested change

	impl<A, B, C> RangeIterator for _Combine<A, B, C>
	where
	A: RangeIterator,
	B: RangeIterator,
	C: Combinator,
	{
	}

You explicitly say this is not satisfied. (I'm still a bit confused about why _Combine is even necessary) You are wrapping it as Combine(Merge(_Combine)) but does this just remove some code duplication or something?

[timvermeulen]

Yes, great catch, that is indeed not satisfied. Combine does implement it, _Combine doesn't.

The _Combine layering is less than ideal, if trait methods could return impl Trait then there'd be no need for a type that wraps Merge<_Combine>. I found that integrating the Merge code into what is now _Combine made it a lot messier, and Merge will indeed be able to be used elsewhere in the future (in unioning multiple bitfields, which is currently implemented very naively).