mozilla::ReverseIterator is required for "stashing iterators", but otherwise
std::reverse_iterator can be used and is potentially more efficient.
Also fix some other glitches in iterator type definitions.
Differential Revision: https://phabricator.services.mozilla.com/D79489
This also uses MakeUniqueForOverwrite* in two places where we immediately copy
over the Buffer from a Span.
Adds move assignment operator as well.
Differential Revision: https://phabricator.services.mozilla.com/D75510