Add RestDestTemp function to simplify ReadField calls that just want to read a field into a new object.
Instead of requiring ReadField callers to construct the object in advance and pass it with ReadDestValue, or to write a custom emplace function and pass it with ReadDestEmplace, they can use RestDestTemp to make ReadField just construct a temporary object internally that can be moved from.
Add test coverage for this. Also add missing documentation comments for ReadDestEmplace and ReadDestValue classes.