General decode/encode functions

[jordy25519]

The doughnut encode/decode methods are coupled (one and the same) with the parity-scale-codec trait.

It would be better to provide independent encode/decode functions which the parity-scale-codec impls can simply call into.
The parity-scale-codec use has some implications around the input buffer type and errors types which make it not the best API for general use outside of substrate.

The general functions could enable things like:

• extraneous bytes checks on decoding as an option
• encoding with 0 domains as error (currently fails silently encodes nothing, which is confusing)