Fixed a bug in `values` within file lazy.rkt.

The issue was that when `values` was used with a single input, that input was being forced too early.
Now code such as:
(! (letrec-values ([(x) (values (list y))] [(y) (values 1)]) (car x))  )
should produce 1 instead of #<undefined>.

Some simple test cases were also added.