Convert most unchecked size/index `as` casts to checked casts with
`TryFrom`, included another top-level error variant.
This has several motivations:
- Making the library as robust as possible, including to platform
differences.
- Unchecked casts are susceptible to breakage during refactorings.
- Preparing for BigTIFF support, which will require the introduction of
generics and associated types for some parts of the de/encoders.
Unchecked primitive casts cannot be bound, while TryFrom can, and
overflowing conversions become more likely.
- Remaining u16 casts may be prone in failure in existing practice and
not just in theory during future changes.
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.
The discriminant of some enum types can be retrieved with the `as`
operator and in C style enums it is also possible to set a specific
discriminant. However, that may silently change—breaking the value
written to the file. It is better to use that technique mostly for ffi
purposes.
Farkal