ast: Add Fold, Folder traits, reorganize visit

parser: fix precedence of `BindOp::Let`

src/ast.rs

@@ -4,6 +4,8 @@ pub mod macro_matcher;
 
 pub mod visit;
 
+pub mod fold;
+
 /// An annotation: extra data added on to important AST nodes.
 pub trait Annotation: Clone + std::fmt::Display + std::fmt::Debug + PartialEq + Eq {}
 impl<T: Clone + std::fmt::Debug + std::fmt::Display + PartialEq + Eq> Annotation for T {}
@@ -130,6 +132,7 @@ pub enum Op {
     XorSet, // Expr ^= Expr
     OrSet,  // Expr |= Expr
 }
+
 /// A qualified identifier
 ///
 /// TODO: qualify identifier