Implement RangeEnd, Range

[sgwilym]

Implements Willow’s one-dimensional ‘range’ grouping as described here, with testing for value inclusion, range inclusion, intersection creation, and a method which creates a range including all values of that type.

While it's still possible to create empty ranges by initialising a Range directly, attempting to do so with new_closed will return a EmptyRangeError.

This is a draft while I add some fuzzing tests, but I think the implementation is ready to look at!

[Frando]

Looks good! And cool that my code from iroh-willow is of direct use here :-)

[AljoschaMeyer]

Btw, I think that all functions that deal with potentially empty ranges should either use Option or Result<Range, RangeError>, but no mixture (currently, intersecting yields an Option, but closed creation a Result).

[sgwilym]

@AljoschaMeyer sure. I think it makes more sense for creation to be an Option than to make non-intersection return an error, on it.