Add the cbor!() macro

[npmccallum]

The cbor!() macro is inspired by serde_json::json!(), but the
implementation is simpler and the macro should be more useful. In
particular, the cbor!() macro returns Result<Value, Error>, so it can be
used in places that need careful error handling.

[npmccallum]

One decision I was on the fence about: cbor!(null) or cbor!(Null). The first one is closer to JSON and matches other keywords like true and false. The second one is closer to the Rust name. I could go either way. Let me know your preference.

[pyfisch]

This could be a separate crate, right?