Codec API Redesign

As seen in #299, having verification in the codec (#253) does not fit nicely into the current design. The codec (i.e. Encoder/Decoder, descriptors, etc) is currently designed around simplicity of use, so we use exceptions to trigger expected, but invalid behaviour (such as validation errors).

To have richer error types (also coming from validation), the API would be best rewritten using `Result` type, so that we can distinguish and handle different error types.