intuitive range building

[pH14]

In #122 (comment) I was surprised to find that RangeOptionBuilder::from(subspace.range()) does not actually build a range over a subspace! This is a tricky gotcha, and we should make the range building as intuitive as possible.

To spitball one possibility: we could create a separate Range trait/struct that's comprised of two KeySelectors, that things like Subspace, Tuple, and, in the future, Directory implement. Rather than RangeOptionBuilder accepting any Encode, perhaps it accepts only Range.