add exp parenthesis syntax (FIXME: shift/reduce conflict!)

lib/syntax/parser.mly

@@ -57,6 +57,7 @@ singleexp:
   | i = Int { Literal (Int i) }
   | p = path { Path p }
   | f = path; a = args { Call (f, a) }
+  | "("; e = exp; ")" { e }
 
 path:
   | x = Name { Var x }