message: [c++] Remove needless std::string creation in bf
  A bare, observing `const char*` is fine for the OpenFile helper
  function, as that's what the underlying `fopen` API uses.
  Closes
diff: using namespace bf;
  -FILE* OpenFile(const std::string& path, const char* mode)
  +FILE* OpenFile(const char* path, const char* mode)
  {
  FILE* file;
  #ifdef _MSC_VER
  +
  // Under the compiler settings we use with MSVC, fopen is not considered
  // "secure", so we use the "secure" variant.
  - errno_t err = fopen_s(&file, path.c_str(), mode);
  + errno_t err = fopen_s(&file, path, mode);
  if (err != 0)
  {
  BOND_THROW(bond::StreamException, "Error " << ErrorString(err) << " opening file " << path);
  @@ -25,7 +26,7 @@ FILE* OpenFile(const std::string& path, const char* mode)
  #else
  - file = fopen(path.c_str(), mode);
  + file = fopen(path, mode);
  if (file == nullptr)
  {
  BOND_THROW(bond::StreamException, "Error " << ErrorString(errno) << " opening file " << path);
  