`RawValue` serde support

[quininer]

I don't like this hack, but it's a useful feature. https://github.com/serde-rs/json/blob/v1.0.107/src/raw.rs#L294
If serde supports downcast objects, that should allow for a more elegant implementation.

[rkuhn]

Not sure exactly which part of “this hack” you refer to. I’m finding myself in need of deserializing a single CBOR item into a RawValue and I think this could be quite simply done by changing the implementation of deserialize_ignored_any to use the decoder of RawValue and feed the byte slice into Visit::visit_bytes instead of visit_unit. Does that sound like a plan?

[quininer]

@rkuhn To support RawValue, we need a way for Deserializer to identify the RawValue type. but since serde does not allow passing &dyn Any, the type must be identified by struct name or thread_local.

I'm not against this hack, I just lack strong motivation to implement it. so, PR are welcome.

edit: Ah I see what you mean, this seems to work, but there is an extra overhead for ignored any. I think this is acceptable for cbor.