ast: Give for loops a dedicated node and remove eager desugar

src/ast.rs

@@ -184,6 +184,7 @@ pub enum Use {
 /// fn     Pat Expr
 /// mod    Pat Expr
 /// impl   Pat Expr
+/// for    Pat in Expr Expr (else Expr)?
 /// Pat => Expr // in match
 /// ```
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -214,6 +215,8 @@ pub enum BindOp {
     Struct,
     /// An enum definition
     Enum,
+    /// A `for Pat in Expr Expr (else Expr)?` binding
+    For,
     /// A `Pat => Expr` binding
     Match,
 }