[C++ bindings] Add move-assign operator, require rvalue for move

[C++ bindings] Add move-assign operator, require rvalue for move

This adds a move-assignment operator (`A& operator=(A&& o)`) to our
C++ wrapper classes as well as requiring an rvalue for the move
auto-convert operator (`operator CStruct()() &&`).

The second makes the C++ wrapper classes much easier to work with
by requiring an explicit `std::move` when the bindings will
automatically move a C++-wrapper object into a C object.