Support printing values without Abseil #1531

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.