Improvements of asn1scc (WUT)

In order to iterate on this - and define priorities, I summarize here the list of issues reported by WUT:

1. Missing/Incomplete documentation (we have no real end-user documentation of the compiler (apart from the ACN part).)
2. Management of errors :

* [DONE] Lack of automatic constraints check in Decode functions (Add call to isConstraintValid after decoding)
* Usage of assert in asn1scc generated functions
* [DONE] Returning of unnamed / undefined error codes

1. Misleading initialisation of a kind attribute for ASN.1 CHOICE construction (Some confusion with the NONE choice)

2. Bitstream structure issues (arious issues, such as the meaning of the "count" field which is different for encoding and decoding)

3. Not sufficient model consistency checks

4. Ambiguity in the definition of the required buffer sizes for encoding ("+1 for zero-sized type" is not documented)

5. Quality of the asn1scc-generated test suite