Alter tests for new semantics for RETURN_VALS
Initial implementation, with some issues
Get multi-value replace working.

This is not totally spec compliant yet (some renaming has to happen) and the JS
driver in atomic_get_set is experiencing some wonky nondeterminism that results
in occasional test failures.  It is not suitable for release until both these
are rectified and the code uses return_changes instead of return_vals
consistently.
Remove debug.hpp which we don't use
Accomodate reviewer requests
Replace return_vals with return_changes throughout codebase
Change return value to changes instead of values
Add return_vals error message
Merge branch 'gchpaco_1382' into next