Replace variadic List constructor with one taking a initializer_list

List provides a variadic constructor currently implemented with some preprocessor magic. Given that we already require C++11 we can replace that one with a much simpler one just taking a std::initializer_list. This would change the invocations,

auto l1 = List<int>(1, 2, 3);    // now
auto l2 = List<int>({1, 2, 3});  // proposed

This addresses an existing TODO.