[Feature] Coresponding Serializer for Box<RawValue>

[oxalica]

RawValue is just a pre-serialized string and should be considered a data format just like Value. Currently the only way to construct Box<RawValue> is through serde_json::value::to_raw_value. It is difficult to write functions using intermediate types to be general over Box<RawValue> and Value, while allowing serializing arbitrary impl Serialize into it.

Thus I propose to have a struct RawSerializer: Serializer<Ok = Box<RawValue>> similar to serde_json::value::Serializer. So we can have fn func<IntermRepr: Serializer> with func::<value::RawSerializer> and func::<value::Serializer>.