Refactor random number generation to factory

- Use a `make-random` factory to create isolated random number
  generators.
- Remove the global `prng` from the `Environment`.
- Ensure `make-random` can be called with or without a seed.

examples/pipeline_script_ticker.myc

@@ -1,10 +1,10 @@
-;; Benchmark: 2.0us
-;; Benchmark-Repeat: 1015
+;; Benchmark: 3.0us
+;; Benchmark-Repeat: 593
 ;; Output: PipelineNode[last: Some(110.45243843391206)]
 
 (do
   ;; Set the random seed to match the existing test output exactly
-  (seed! 42)
+  (def random (make-random 42))
 
   ;; Use our new generic create-ticker to pulse 3 times
   (def cnt 3)