Add enum RawKind and is_* to RawValue

[bheylin]

I mentioned the proc-macro based JSON processor I'm using in PR #1221. This processor ultimately converts the JSON to a Deserializable Rust struct. While converting, it has specialized null handling and custom Errors for missing fields and such.

To accomplish this I currently use an extension trait to provide methods that report what JSON kind (null, bool, string, etc) the RawValue contains and a set of tests to assert the invariants of a RawValue. Invariants such as:

• A RawValue has whitespace stripped during deserialization
• An empty string can't be deserialized into a RawValue.

I think knowing the type of a RawValue can be useful to all users of serde_json.

What do you think?

[Marcono1234]

Value has similar is_... methods as well. Maybe there could be a common ValueKind (or similar) used for both, instead of adding this specific RawKind (and risking that it needs to be duplicated / renamed in the future)?

(This is just an idea from a drive-by user though; would be good to wait for the opinion of the maintainer before acting on this.)