This allows us to add an API for accessing an undefined-value/structure
directly on ValueContext.
- ifd_tag_entry.go: Fixed several methods on `IfdTagEntry` to be by-
reference. It was a latent bug that they weren't.
- value_context.go: Added `newValueContextFromTag` to streamline
the changes above.
The upshot of this is that generating a list of type-correct slices or a
string from a tag's value is now stupidly easy.
- Broke parsing into an independent type (parser.go:Parser).
- Moved primary value-processing logic to `ValueContext` so that it
coexists with the actual data (though as much basic functionality as
possible is implemented independently and reused here). This
eliminates extremely ridiculously obtuse usage procedure.
- Deprecated almost all existing TagType functionality (in order to
distance us from this now-legacy usage pattern). Existing
functionality maintained for now. We'll drop it when we do the next
release major.
- `ValueContext` is now passed by reference.
Added a `Size()` method to it and decommissioned redundant
`TagTypeSize()` and `(TagType).Size()`.
- ifd_enumerate.go: Moved `ValueContext` definition to
value_context.go .
Dustin Oprea