message: Switch from idiomatic to performant way to read a file.
  Reading large generated files showed that GetContent() was super slow.
  With this, a 245MiB file reading from a local filesystem is
  sped up from 11 seconds -> 0.045 seconds.
diff: @@ -144,11 +144,18 @@ absl::Status GetContents(absl::string_view filename, std::string *content) {
  absl::Status usable_file = FileExists(filename_str);
  if (!usable_file.ok()) return usable_file; // Bail
  fs.open(filename_str.c_str());
  + std::error_code err;
  + const size_t prealloc = fs::file_size(filename_str, err);
  + if (err.value() == 0) content->reserve(prealloc);
  stream = &fs;
  }
  if (!stream->good()) return CreateErrorStatusFromErrno("can't read");
  - content->assign((std::istreambuf_iterator<char>(*stream)),
  - std::istreambuf_iterator<char>());
  + char buffer[4096];
  + while (stream->good() && !stream->eof()) {
  + stream->read(buffer, sizeof(buffer));
  + content->append(buffer, stream->gcount());
  + }
  +
  // Allow stdin to be reopened for more input.
  if (use_stdin && std::cin.eof()) std::cin.clear();
  return absl::OkStatus();
  