Implement de::from_slice_count

[ericmarkmartin]

Add a function that deserializes first valid cbor value out of a slice of bytes
and returns the number of consumed bytes.

[ericmarkmartin]

@Firstyear and I would really like this functionality for our work in kanidm/webauthn-rs, where we need to parse cbor values out of byte streams.

[ericmarkmartin]

I'm also somewhat curious about why I have to subtract one off of the error's offset.

[dtolnay]

This use case doesn't strike me as warranting a dedicated API inside serde_cbor. I would prefer to recommend that the caller uses the following code instead, which I find simple/clear.

let mut deserializer = Deserializer::from_slice(slice);
let value = T::deserialize(&mut deserializer)?;
let processed = deserializer.byte_offset();

[Firstyear]

Is there somewhere within the serde_cbor codebase where this could be added as an example usage?

[ericmarkmartin]

Is there somewhere within the serde_cbor codebase where this could be added as an example usage?

Maybe in from_slice? @dtolnay, would you accept a PR adding such an example to the doc string?