Support printing values without Abseil

[tlively]

I'm experimenting with using FuzzTest in https://github.com/webassembly/binaryen. We already use GoogleTest, but we don't use Abseil. Like FuzzTest, GoogleTest supports extensible printing using AbslStringify, but it also supports printing via user implementations of PrintTo or <<. Since our codebase does not use Abseil, implementing AbslStringify for our types is not an attractive option, so it would be great if FuzzTest supported printing via PrintTo or << like GoogleTest. Without this, failing fuzz tests cannot usefully report the arguments that made them fail, which makes the fuzz tests themselves much less useful.

[tlively]

For anyone else in the same situation, here is a catch-all workaround that implements AbslStringify for anything that implements <<:

template<typename T> constexpr bool type_exists = true;

template<typename Sink, typename T>
void AbslStringify(
  Sink& sink,
  const T& val,
  std::enable_if_t<type_exists<decltype(std::cout << val)>, bool> = false) {
  std::stringstream ss;
  ss << val;
  sink.Append(ss.str());
}

Of course it would be nice if we didn't need this workaround :)