Add LhsExpr parsing

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

src/syn/parser.y

@@ -19,7 +19,24 @@ Body -> Result<Vec<Stmt>>:
 
 Stmt -> Result<Stmt>:
         Expr { Ok(Stmt::Expr($1?)) }
-    //|   Assign { todo!() }
+    |   Assign { todo!() }
+    ;
+
+Assign -> Result<AssignStmt>:
+        LhsExpr '=' Expr {
+            todo!()
+        }
+    ;
+
+LhsExpr -> Result<LhsExpr>: 
+        AccessExpr {
+            let lhs = match $1? {
+                Expr::Access(access) => LhsExpr::SetAttr(*access),
+                Expr::Atom(Atom::Ident(local)) => LhsExpr::Local(local),
+                _ => todo!("TODO : invalid syntax, raise error"),
+            };
+            Ok(lhs)
+        }
     ;
 
 Expr -> Result<Expr>: BinExpr { $1 };