Deserialize into lexpr::Value

[laptou]

Hi, this is a cool crate! I'm submitting a feature request, which is the ability to deserialize into lexpr::Value. It would be nice to have this for the same reason that it's useful in serde_json: sometimes you want to validate/parse certain parts of an object, but allow other parts of it to be unstructured data. Right now, I can't find an easy way to do that.