Bug 1645429 - Use std::reverse_iterator instead of mozilla::ReverseIterator where possible. r=froydnj

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