Refactor Call to include is_tail flag

The `TailCall` variant has been merged into `Call` by adding an
`is_tail` boolean field. This simplifies the AST by reducing the number
of node variants and makes it easier to handle tail call optimization.
The `tco.rs` module is responsible for setting this flag when a call
occurs in tail position.

src/ast/compiler/bound_nodes.rs

@@ -83,11 +83,7 @@ pub enum BoundKind<T = ()> {
     Call {
         callee: Box<BoundNode<T>>,
         args: Box<BoundNode<T>>,
-    },
-
-    TailCall {
-        callee: Box<BoundNode<T>>,
-        args: Box<BoundNode<T>>,
+        is_tail: bool,
     },
 
     Block {
@@ -145,11 +141,8 @@ where T: PartialEq {
              BoundKind::Lambda { params: pb, upvalues: ub, body: bb, positional_count: pcb }) => {
                 pa == pb && ua == ub && ba == bb && pca == pcb
             }
-            (BoundKind::Call { callee: ca, args: aa }, BoundKind::Call { callee: cb, args: ab }) => {
-                ca == cb && aa == ab
-            }
-            (BoundKind::TailCall { callee: ca, args: aa }, BoundKind::TailCall { callee: cb, args: ab }) => {
-                ca == cb && aa == ab
+            (BoundKind::Call { callee: ca, args: aa, is_tail: ta }, BoundKind::Call { callee: cb, args: ab, is_tail: tb }) => {
+                ca == cb && aa == ab && ta == tb
             }
             (BoundKind::Block { exprs: ea }, BoundKind::Block { exprs: eb }) => {
                 ea == eb
@@ -191,8 +184,7 @@ impl<T> BoundKind<T> {
                 };
                 format!("LAMBDA({}, Captures:{})", p_str, upvalues.len())
             },
-            BoundKind::Call { .. } => "CALL".to_string(),
-            BoundKind::TailCall { .. } => "T-CALL".to_string(),
+            BoundKind::Call { is_tail, .. } => if *is_tail { "T-CALL".to_string() } else { "CALL".to_string() },
             BoundKind::Block { .. } => "BLOCK".to_string(),
             BoundKind::Tuple { elements } => format!("TUPLE({})", elements.len()),
             BoundKind::Record { fields } => format!("RECORD({})", fields.len()),