Originally I was using just `(usize, usize)` for marks, and `Vec`s
of those for sets. But that was already becoming unwieldy even for
really basic code. This abstracts just a handful of common
operations away to make this easier to reason about. However,
the internals are (intentionally) left exposed to allow other fiddly
things to be done.
This is very WIP. The intent is for these to be used for e.g.
cursors, screen positions, etc. The intent is for the buffer
itself to keep them synced properly with the actual text on edits,
although this is not yet implemented.
