* module/language/cps/dfg.scm (constant-needs-allocation?): Use of a
constant in a $values expression of any arity does not cause slot
allocation.
* module/language/cps/compile-bytecode.scm (compile-fun): Allow $values
with a constant value to be compiled in test context. Really we
should fold these in a previous pass!
* module/language/cps/slot-allocation.scm (allocate-slots): Don't
allocate slots to unused results of function calls. This can allow us
to avoid consing a rest list for call-with-values with an ignored rest
parameter, and can improve the parallel move code.
* module/language/cps/compile-bytecode.scm (compile-fun): Adapt to avoid
emitting bind-rest in values context if the rest arg is unused.
