Add positional_count field to Lambda

This field is used for static optimization, determining if parameters
are purely positional.

src/ast/compiler/bound_nodes.rs

@@ -72,10 +72,12 @@ pub enum BoundKind<T = ()> {
     },
 
     Lambda {
-        params: Box<BoundNode<T>>,
+        params: Rc<BoundNode<T>>,
         // The list of variables captured from enclosing scopes
         upvalues: Vec<Address>, 
         body: Rc<BoundNode<T>>,
+        /// Static optimization: number of positional parameters if the pattern is flat.
+        positional_count: Option<u32>,
     },
 
     Call {