automatic derivation of `instance Codec`?

[jwaldmann]

(As #45, this is a request for comments)

Can we automate instance Codec a in some cases? Examples: if Decoded a is ...

• a tuple, we just concatenate the encodings of components
• a finite enumeration, we encode the index of the constructor
• any (non-recursive) algebraic data type, we encode the index of the constructor, and then overlap the encoding of the argument tuples.

Example: https://gitlab.imn.htwk-leipzig.de/waldmann/misc/tree/master/cal/2019/12#room-for-improvement

Perhaps Decoded is going the wrong way for this: my goal is that I don't want to see the encoded type in the source, as it should be computed from the original (decoded) type.