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.

src/ast/environment.rs

@@ -28,7 +28,6 @@ pub struct Environment {
     pub global_types: Rc<RefCell<HashMap<GlobalIdx, StaticType>>>,
     pub global_purity: Rc<RefCell<HashMap<GlobalIdx, Purity>>>,
     pub global_values: Rc<RefCell<Vec<Value>>>,
-    pub prng: Rc<RefCell<fastrand::Rng>>,
     pub function_registry: Rc<RefCell<HashMap<GlobalIdx, BoundNode>>>,
     pub typed_function_registry: Rc<RefCell<HashMap<GlobalIdx, AnalyzedNode>>>,
     pub monomorph_cache: Rc<RefCell<MonoCache>>,
@@ -102,7 +101,6 @@ impl Environment {
             global_types: Rc::new(RefCell::new(HashMap::new())),
             global_purity: Rc::new(RefCell::new(HashMap::new())),
             global_values: Rc::new(RefCell::new(Vec::new())),
-            prng: Rc::new(RefCell::new(fastrand::Rng::new())),
             function_registry: Rc::new(RefCell::new(HashMap::new())),
             typed_function_registry: Rc::new(RefCell::new(HashMap::new())),
             monomorph_cache: Rc::new(RefCell::new(HashMap::new())),