Several other uses that currently rely on num-traits for conversion have
no Unknown representation. Thus, the converter now outputs only known
variants or an error and the exposing function then can put the error
value into an Unknown variant instead.
Factor out parts that are unique to instantiations such as the name, the
documentation, the underlying type, the public conversion methods.
Also adds the ability to document individual items and remodels the
syntax to more closely resemble ordinary enum definitions.
There are no function changes from 0.3 but the compiler requirements
have been lowered. This avoids having to make a decision on `image`
right now, which would delay valuable bug fixes and improvements.
Andreas Molzer