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/ast/vm.rs

@@ -361,6 +361,25 @@ impl VM {
         dispatch_eval!(self, node, eval)
     }
 
+    /// Resolves potential tail call requests iteratively until a final value is reached.
+    pub fn resolve_tail_calls(&mut self, mut result: Value) -> Value {
+        while let Value::TailCallRequest(payload) = result {
+            let (next_obj, next_args) = *payload;
+            if let Some(closure) = next_obj.as_any().downcast_ref::<Closure>() {
+                result = match self.run_with_args(closure, next_args) {
+                    Ok(v) => v,
+                    Err(e) => panic!("Myc Runtime Error (TailCall): {}", e),
+                };
+            } else {
+                panic!(
+                    "Tail call target is not a closure: {}",
+                    next_obj.type_name()
+                );
+            }
+        }
+        result
+    }
+
     fn eval_observed<O: VMObserver>(
         &mut self,
         observer: &mut O,