RawValue support like in serde_json

[drewhk]

serde_json has the ability to preserve chunks of unparsed JSON in a RawValue (which can either borrow a slice of the original input or box+copy the bytes verbatim). This is very useful for low-level infrastructure code that pushes envelopes around only parsing part of the message and not the payload itself. I tried to implement this for CBOR using the high level APIs, but it seems like this is impossible without hacking serializer directly as there is no API to write bytes back directly on the serializer.

[CobaltCause]

Wow, this was opened a year ago. Anyway I just ran into needing this feature myself, so I took a stab at implementing it in #211, thought I'd comment in case you or anyone else were still interested in this issue.