Introduce Purity enum for optimizer

Refactor the optimizer to use a Purity enum instead of a boolean for
tracking function purity. This allows for a more granular representation
of purity:

- `Impure`: Functions with side effects.
- `SideEffectFree`: Functions without side effects but may not be
  deterministic (e.g., `now()`, `random()`).
- `Pure`: Functions without side effects and are deterministic.

This change enhances the optimizer's ability to perform more aggressive
optimizations by accurately determining function purity.

src/ast/environment.rs

@@ -12,7 +12,7 @@ use crate::ast::compiler::bound_nodes::{Address, BoundNode};
 use crate::ast::compiler::dumper::Dumper;
 use crate::ast::compiler::lambda_collector::LambdaCollector;
 use crate::ast::compiler::macros::{MacroEvaluator, MacroExpander, MacroRegistry};
-use crate::ast::compiler::optimizer::Optimizer;
+use crate::ast::compiler::optimizer::{Optimizer, Purity};
 use crate::ast::compiler::specializer::{FunctionRegistry, MonoCache, Specializer};
 use crate::ast::compiler::tco::{ExecNode, TCO};
 use crate::ast::rtl;
@@ -21,7 +21,7 @@ use crate::ast::rtl::intrinsics;
 pub struct Environment {
     pub global_names: Rc<RefCell<HashMap<Symbol, u32>>>,
     pub global_types: Rc<RefCell<HashMap<u32, StaticType>>>,
-    pub global_purity: Rc<RefCell<HashMap<u32, bool>>>,
+    pub global_purity: Rc<RefCell<HashMap<u32, Purity>>>,
     pub global_values: Rc<RefCell<Vec<Value>>>,
     pub function_registry: Rc<RefCell<HashMap<u32, BoundNode>>>,
     pub typed_function_registry: Rc<RefCell<HashMap<u32, TypedNode>>>,
@@ -116,7 +116,7 @@ impl Environment {
         &self,
         name: &str,
         ty: StaticType,
-        is_pure: bool,
+        purity_level: Purity,
         func: impl Fn(Vec<Value>) -> Value + 'static,
     ) {
         let mut names = self.global_names.borrow_mut();
@@ -127,7 +127,7 @@ impl Environment {
         let idx = values.len() as u32;
         names.insert(Symbol::from(name), idx);
         types.insert(idx, ty);
-        purity.insert(idx, is_pure);
+        purity.insert(idx, purity_level);
         values.push(Value::Function(Rc::new(func)));
     }
 
@@ -140,7 +140,7 @@ impl Environment {
         let idx = values.len() as u32;
         names.insert(Symbol::from(name), idx);
         types.insert(idx, ty);
-        purity.insert(idx, true); // Constants are always pure
+        purity.insert(idx, Purity::Pure); // Constants are always pure
         values.push(val);
     }