Interval trees

These are useful: https://en.wikipedia.org/wiki/Interval_tree
There's some overlap with RangeSet. I'd want interval sets/dictionaries as first-class components, but you could get better complexity bounds by switching to interval tree representation when RangeSets get large (for non-overlapping ranges).

Start by reading the linked interval tree overview and comparing the proposal with RangeSet. Clarify whether the goal is first-class interval sets and dictionaries, a large-RangeSet representation change, or both; done requires an agreed design and complexity bounds before implementation can be scoped.

Written by the indexing model from the issue text.