Extract calculate_stack_size to a standalone function

The `calculate_stack_size` method was duplicated in `bound_nodes.rs` and
`tco.rs`. This commit extracts it into a single standalone function in
`tco.rs` to avoid duplication. The stack size is now calculated and
stored in the `ExecNode`'s `ty` field during the TCO optimization phase.

src/ast/vm.rs

@@ -141,8 +141,8 @@ impl VM {
         }
     }
 
-    pub fn run(&mut self, root: &ExecNode, stack_size: u32) -> Result<Value, String> {
-        self.run_with_observer(&mut NoOpObserver, root, stack_size)
+    pub fn run(&mut self, root: &ExecNode) -> Result<Value, String> {
+        self.run_with_observer(&mut NoOpObserver, root)
     }
 
     pub fn resolve_tail_calls<O: VMObserver>(
@@ -263,12 +263,11 @@ impl VM {
         &mut self,
         observer: &mut O,
         root: &ExecNode,
-        stack_size: u32,
     ) -> Result<Value, String> {
         self.stack.clear();
         self.frames.clear();
 
-        self.stack.resize(stack_size as usize, Value::Void);
+        self.stack.resize(root.ty.stack_size as usize, Value::Void);
 
         self.frames.push(CallFrame {
             stack_base: 0,