monads: Fix 'liftN' fallback case.

Reported by Andy Wingo <wingo@igalia.com>.

* guix/monads.scm (define-lift) <fallback case>: Add missing #'.  Remove
  extra formal parameter.
* tests/monads.scm ("lift"): Add test with 'lift1' as a procedure.

guix/monads.scm

@@ -239,7 +239,7 @@ CONDITION is true, return *unspecified* in the current monad."
             (identifier? #'id)
             ;; Slow path: Return a closure-returning procedure (we don't
             ;; guarantee (eq? LIFTN LIFTN), but that's fine.)
-            (lambda (liftn proc monad)
+            #'(lambda (proc monad)
                 (lambda (args ...)
                   (with-monad monad
                     (return (proc args ...))))))))))))

tests/monads.scm

@@ -95,10 +95,12 @@
 
 (test-assert "lift"
   (every (lambda (monad run)
-           (let ((f (lift1 1+ monad)))
+           (let ((f (lift1 1+ monad))
+                 (g (apply lift1 1+ (list monad))))
              (with-monad monad
                (let ((number (random 777)))
                  (= (run (>>= (return number) f))
+                    (run (>>= (return number) g))
                     (1+ number))))))
          %monads
          %monad-run))