feat(xatu): add let order rule

src/xatu/ast.rs

@@ -29,6 +29,7 @@ impl std::fmt::Display for SourceRange {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum NodeData {
     Binding(Binding),
+    LetBlock(LetBlock),
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -37,6 +38,18 @@ pub struct Binding {
     pub value: Expr,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct LetBlock {
+    pub bindings: Vec<LetBinding>,
+    pub body_references: Vec<Ident>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct LetBinding {
+    pub name: Ident,
+    pub references: Vec<Ident>,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Ident {
     pub name: String,