Implementing Value type

[ecton]

This PR adds implementations of:

• Parsing Rsn into a Value without serde
• Displaying a Value without serde
• "Parsing" a Serialize into a Value
• "Deserializing" a Value into a Deserialize

The Value type in this commit becomes a "dumb" value type, which doesn't support comments or whitespace or annotations. This is meant to be a lightweight anonymous data interchange representation. A separate type will be added to represent an Rsn document "losslessly".

• Implement error handling

[ecton]

There's one TODO regarding option handling. I left it as I think it should -- enabling Some to be implied by having a value be present. This allows having existing files that didn't have an Option in a specific field being able to parse without updating the file to wrap the value in Some().

[ModProg]

Yeah Option is a bit of an interresting thing. rsn has first class enums, so it makes sense to use them. But on the other hand there are also caseses were you'd want (both for ser and des) to encode Some as the presence of a value and None as the absense. Not sure if there would be a good way to encode this through serde. The None case can be done through #[serde(skip_serializing_if = "Option::is_none")] but not sure if something similar can be done for Some. There could be a global setting when calling the serializer, but maybe we could also provide a #[serde(with = ...)] module.