Coecisting fractional and persistent read-only ownership

As part of !486 (merged), @tchajed and @simonfv raised the point that sometimes it would be useful to convert ownership of some fraction of a map element to persistent read-only ownership. Right now, our encoding through frac * agree T + agree T (or equivalently (frac + ()) * agree T) requires ownership of the full fraction for that move.

I think such a construction is possible, but it requires #257 (closed). Then we could relate an authoritative map to a fragment that's more like option (frac * agree T) * option (agree T), and ensure that the second option is None unless the sum of all fraction fragments is less than 1.