Refactor instantiate to return Closure

The `instantiate` method in `Environment` has been refactored to return
`Result<Rc<Closure>, String>` instead of `Rc<NativeFunction>`. This
change separates the creation of a `Closure` from its execution,
allowing the caller to manage the `VM` lifecycle and choose the
appropriate execution strategy (e.g., single run vs. repeated benchmark
iterations).

The `NativeFunction` type is now exclusively used for true Rust
intrinsics. The benchmark runner has also been updated to reuse a single
`VM` across iterations, improving performance by avoiding repeated VM
allocations.

tests/closures.rs

@@ -18,8 +18,8 @@ fn test_closure_modification_from_source() {
         .into_result()
         .expect("Failed to compile");
     let linked = env.link(compiled);
-    let func = env.instantiate(linked);
-    let result: Result<Value, String> = Ok((func.func)(&[]));
+    let closure = env.instantiate(linked).expect("Failed to instantiate");
+    let result = env.create_vm().run_with_args(closure, &[]);
 
     match result {
         Ok(Value::Int(20)) => (),