Revise SE-270 proposal for re-review.

These changes are in response to feedback from the SE-270 proposal review.
Removed the `RangeSet` methods that used a collection to convert individual
indexes or range expressions into concrete ranges before performing
operations. Added parameter labels to `RangeSet.insert` and `remove` to
reduce confusion around semantics. Removed the `shift...` methods; these
will be considered in a future proposal.