Add an automatic casting vector (and array)

[JohanMabille]

The use case if for xoptional_sequence: we want to use an uncompressed storage for booleans, so xdynamic_bitset is not suitable. We cannot use vector<bool> because this specialization actually has a compressed storage. So the best option appears to be std::vector<uint_8>, however the boolean value_type must be a boolean and we want to avoid warnings about cast.