message: Only import optional and nullopt into cpp17
  If we set namespace cpp17 = ..., as before, then we can't set it to something else (for example to akrzemi1 for optional and mpark for variant).
diff: #if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
  #include <optional>
  namespace eos {
  - namespace cpp17 = ::std;
  + namespace cpp17 {
  + using ::std::optional;
  + using ::std::nullopt;
  + }
  }
  #else
  #include "eos/cpp17/detail/akrzemi1_optional.hpp"
  namespace eos {
  - namespace cpp17 = ::akrzemi1;
  + namespace cpp17 {
  + using ::akrzemi1::optional;
  + using ::akrzemi1::nullopt;
  + }
  }
  #endif
  