Add support for .cbor control operator

[KtorZ]

This operator is pretty useful for embedding CBOR in CBOR; and relatively low-hanging fruit to implement as we can recursively re-use the existing validation.

• Implemented validation
• Fixed CDDL parser
• Added few unit tests
• Updated README.md

Note
Since the validation rely on serde_cbor, I've only conditionally included it. If the library is imported without that feature, then the behavior is more or less the current one (i.e. a Validation failure for an unsupported control operator).

[ericseppanen]

Thanks for this!

Looks like cargo build --no-default-features --features serde_json needs fixing, but other than that everything looks good.

[KtorZ]

Indeed! I thought that the inline cfg! macro would do the trick but it did not. Falling back to a conditional function overloading work as expected it seems.

[ericseppanen]

I published 1.6.2. Thanks again!