`Yaml.Value` cannot stringify itself

If you attempt to stringify a struct with a `Yaml.Value` in it, it attempts to encode it as a `Yaml.Value { .map = ... }` and printing that instead of just simply stringifying the `Yaml.Value` struct.

I think looking at the `std.json.Stringify.write()` and its companion `std.json.Value.jsonStringify()` functions may give a bit more of an idea on how to solve this?