Support printing values without Abseil

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](https://google.github.io/googletest/advanced.html#teaching-googletest-how-to-print-your-values) 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.