Fix parser for index and call exprs, remove old test that didn't work, add visitor pattern

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/visit.rs

@@ -0,0 +1,24 @@
+use crate::syn::ast::*;
+
+pub trait Accept<V: Visit + ?Sized> {
+    fn accept(&self, visitor: &mut V) -> V::Out;
+}
+
+pub trait DefaultAccept<V: Visit + ?Sized> {
+    fn default_accept(&self, visitor: &mut V) -> V::Out;
+}
+
+pub trait Visit {
+    type Out;
+
+    fn visit_stmt(&mut self, stmt: &Stmt) -> Self::Out;
+    fn visit_assign_stmt(&mut self, assign: &AssignStmt) -> Self::Out;
+    fn visit_lhs_expr(&mut self, lhs_expr: &LhsExpr) -> Self::Out;
+    fn visit_expr(&mut self, expr: &Expr) -> Self::Out;
+    fn visit_bin_expr(&mut self, expr: &BinExpr) -> Self::Out;
+    fn visit_un_expr(&mut self, expr: &UnExpr) -> Self::Out;
+    fn visit_call_expr(&mut self, expr: &CallExpr) -> Self::Out;
+    fn visit_index_expr(&mut self, expr: &IndexExpr) -> Self::Out;
+    fn visit_access_expr(&mut self, expr: &AccessExpr) -> Self::Out;
+    fn visit_atom(&mut self, atom: &Atom) -> Self::Out;
+}