Rework the resource pool implementations

Don't rely on resource equality for keeping track of resources and
make some other tweaks.

tests/resource-pool.scm

@@ -1,9 +1,33 @@
 (use-modules (tests)
              (fibers)
+             (fibers channels)
              (unit-test)
              (knots parallelism)
              (knots resource-pool))
 
+(run-fibers-for-tests
+ (lambda ()
+   (let ((parallelism-limiter (make-parallelism-limiter
+                               1)))
+     (with-parallelism-limiter parallelism-limiter
+                               #f)
+
+     (destroy-parallelism-limiter parallelism-limiter))))
+
+(run-fibers-for-tests
+ (lambda ()
+   (let ((parallelism-limiter (make-parallelism-limiter
+                               1))
+         (channel
+           (make-channel)))
+     (spawn-fiber
+      (lambda ()
+        (with-parallelism-limiter parallelism-limiter
+           (put-message channel #t)
+           (sleep 1))))
+     (get-message channel)
+     (destroy-parallelism-limiter parallelism-limiter))))
+
 (define new-number
   (let ((val 0))
     (lambda ()