feature wish(imap-types): iterator on simplified+cleanup ranges in a SequenceSet

Currently, `SequenceSet::iter()` allows iterating on the individual indexes that the `SequenceSet` represents.
I would like to be able to iterate on the ranges represented by the `SequenceSet` before they get expanded into indices, but after they are simplified and cleaned up.
The motivation is to be able to perform efficient range queries when the underlying storage supports it.

If this sounds reasonable I can try to volunteer an implementation.