Make resource pool changes and add parallelism limiter

This was motivated by trying to allow for completely cleaning up
resource pools, which involved removing their use of fiberize which
currently has no destroy mechanism.

As part of this, there's a new parallelism limiter mechanism using
resource pools rather than fibers, and also a fixed size resource
pool.

The tests now drain? and destroy the resource pools to check cleaning
up.

tests/parallelism.scm

@@ -111,4 +111,16 @@
 
      (assert-equal a 1))))
 
+(run-fibers-for-tests
+ (lambda ()
+   (let ((parallelism-limiter (make-parallelism-limiter 2)))
+     (fibers-for-each
+      (lambda _
+        (with-parallelism-limiter
+         parallelism-limiter
+         #f))
+      (iota 50))
+
+     (destroy-parallelism-limiter parallelism-limiter))))
+
 (display "parallelism test finished successfully\n")