Minor refactoring: hide multiple-value vs. single-value distinction inside RETURN-EXP so it presents one interface to the outside world.
Minor refactoring: hide multiple-value vs. single-value distinction
inside RETURN-EXP so it presents one interface to the outside world.

This makes the implementation of source mappings a little easier.

vsedach/Parenscript,fredericksilva/Parenscript