message: Add Slice::stringView() method
diff: #include <functional>
  #include <type_traits>
  +#if __cplusplus >= 201703L
  +#include <string_view>
  +#define VELOCYPACK_HAS_STRING_VIEW 1
  +#endif
  +
  #include "velocypack/velocypack-common.h"
  #include "velocypack/Exception.h"
  #include "velocypack/Options.h"
  @@ -710,6 +715,12 @@ class Slice {
  throw Exception(Exception::InvalidValueType, "Expecting type String");
  }
  +#ifdef VELOCYPACK_HAS_STRING_VIEW
  + std::string_view stringView() const {
  + StringRef ref = this->stringRef();
  + return std::string_view(ref.data(), ref.size());
  + }
  +#endif
  // return the value for a Binary object
  uint8_t const* getBinary(ValueLength& length) const {
  