Refactor native function registration and instantiation

Introduces `register_native` for direct registration of `NativeFunction`
and `register_native_fn` for convenience from closures. The
`Environment::run`
method is removed, and its functionality is now handled by
`Environment::instantiate`,
which packages the linked AST into an invokable `NativeFunction`. This
streamlines
the execution path and better separates compilation/linking from runtime
execution.

src/utils/tester.rs

@@ -123,10 +123,11 @@ pub fn run_benchmarks(update: bool) -> Vec<BenchmarkResult> {
                 let env = Environment::new();
                 // Link once per sample
                 let linked = env.link(node.clone());
+                let func = env.instantiate(linked);
                 let mut total = Duration::ZERO;
                 for _ in 0..n {
                     let start = Instant::now();
-                    let _ = env.run(&linked)?;
+                    let _ = (func.func)(vec![]);
                     total += start.elapsed();
                 }
                 Ok(total)