Fix CDDL compile errors and improve formatting of CCF specs document

[turbolent]

• Add language to code fences (e.g. for JSON)
• Move CDDL to separate file and fix problems reported by https://github.com/anweiss/cddl:
  • + is not a sequencing operator according to the CDDL spec, sequencing seems to be just the default operation (no operator)
  • According to the spec, CBOR tags unfortunately need to be numeric, the syntax is required to be #6.nnn, the tag cannot be symbolic. Still, define names and refer to them in comments
  • Tag content cannot be a name/value pair, see change to CCF_TypeRef

Maybe we can run cddl compile-cddl --cddl ccf.cddl on CI?

[fxamacker]

Great work! 👍. Some thoughts:

• Really love your suggestion to use CI to test CDDL and thanks for suggesting https://github.com/anweiss/cddl

• CDDL definition is the heart of the specs, so I would prefer we keep the CDDL inside a single markdown file and use CI to automatically extract and test the CDDL.

We can add a pair of CDDL comments to make it easy for CI to extract CDDL using sed or grep easier:

;CDDL-BEGIN
... 
;CDDL-END 

I can push a commit to do it this way unless you have any objections. Thoughts?

[turbolent]

@fxamacker Sounds good! Moved the CDDL back