`geiser-eval-region` does not return expected values

Hi all!

I was having some trouble producing results when evaluating org-mode source blocks with ob-scheme.el correctly and I think I found the cause.

8bc53c16 changes the return of geiser-eval-region to the output of tq-enqueue (deeper in the stack trace) and I'm not sure if this is the intended effects. The output is still being correctly printed by the expression (message "%s" res) in the closure, though.