Refactor Purity enum and NativeFunction struct

Move `Purity` enum definition from `optimizer.rs` to `types.rs` and
create a `NativeFunction` struct to hold the function and its purity.
Update `Value::Function` to store `Rc<NativeFunction>` and
`Value::make_function`
helper to simplify creation.

This change allows tracking the purity of native functions, which is
useful
for optimization and static analysis.

src/ast/environment.rs

@@ -2,7 +2,6 @@ use crate::ast::compiler::binder::Binder;
 use crate::ast::compiler::{TypeChecker, TypedNode};
 use crate::ast::nodes::{Node, Symbol, UntypedKind};
 use crate::ast::parser::Parser;
-use crate::ast::types::{Object, StaticType, Value};
 use crate::ast::vm::{TracingObserver, VM};
 use std::cell::RefCell;
 use std::collections::HashMap;
@@ -12,11 +11,12 @@ 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, Purity};
+use crate::ast::compiler::optimizer::Optimizer;
 use crate::ast::compiler::specializer::{FunctionRegistry, MonoCache, Specializer};
 use crate::ast::compiler::tco::{ExecNode, TCO};
 use crate::ast::rtl;
 use crate::ast::rtl::intrinsics;
+use crate::ast::types::{Object, Purity, StaticType, Value};
 
 pub struct Environment {
     pub global_names: Rc<RefCell<HashMap<Symbol, u32>>>,
@@ -130,7 +130,7 @@ impl Environment {
         names.insert(Symbol::from(name), idx);
         types.insert(idx, ty);
         purity.insert(idx, purity_level);
-        values.push(Value::Function(Rc::new(func)));
+        values.push(Value::make_function(purity_level, func));
     }
 
     pub fn register_constant(&self, name: &str, ty: StaticType, val: Value) {