Change which method to be a property and return an enum type.

Old code using which() will still work since the enum type has a
__call__ method.

capnp/helpers/fixMaybe.h

@@ -6,17 +6,6 @@ T fixMaybe(::kj::Maybe<T> val) {
   KJ_IF_MAYBE(new_val, val) {
     return *new_val;
   } else {
-    throw std::invalid_argument("member was null");
+    throw std::invalid_argument("Member was null.");
   }
 }
-
-template<typename T>
-const char * getEnumString(T & val) {
-
-  auto maybe_val = val.which();
-  KJ_IF_MAYBE(new_val, maybe_val) {
-    return new_val->getProto().getName().cStr();;
-  } else {
-    return "";
-  }
-}