Tag representations are not decoder- or encoder-specific, so should live
in their own module accessible to both for reuse. The decoder::ifd
module remains since this module is mainly dedicated to decoding the
IFD.
Note that this adds the LONG8 data type from BigTIFF because of bounds
requiring TiffValue. However, this datatype should not actually be
written to tags.
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.
Andrew Champion