Opaque string typedef in version 1.2.0

Version 1.2.0 adds opaque typedefs for standard strings, one that prevents modifications via const char * and one that provides the complete interface. Find these in opaque::experimental. It also adds a header ostream.hpp to make std::ostream accept opaque typedefs via their underlying_type, and a header hash.hpp providing a macro OPAQUE_HASHABLE that will create a std::hash specialization for an opaque typedef.