- ifd_enumerate: Added utility functions to parse individual, encoded
IFDs and tags.
- ifd_builder: Corrected `NewBuilderTagFromConfig()` to automatically
encode whichever type of value-argument it gets.
- Updated IfdByteEncoder tests to use it instead of hacking-together
their own BT's (makes for more standardized, consistent testing).
- Universally refactored all core IFD knowledge implemented upon a
single IFD name to instead work with IfdIdentity instances, instead,
in order to validate that we only recognize the IFDs only in the
context of the correct parents in the hierarchy.
- Implemented standard testing byte-order (assigned to
TestDefaultByteOrder).
- Using an interface{} required us to validate the encapsulated type or
risk forgetting to.
- AddTagsFromExisting now just copies the bytes verbatim if no value-
resolver provided.
- ifd_builder:
- We now validate that `builderTag` instances are either a byte array
or a child Ifd before allowing to add.
- Renamed `builderTag.valueBytes` to `builderTag.value` (it's not
necessarily bytes).
- `builderTag` now gets the child IFD name if representing a child
IFD.
- Added ability to dump structure to string slice.
- Added NewIfdBuilderFromExistingChain to reconstruct whole chain
from a read IFD chain (collection).
- Wrote IB dumper.
- Added structural tests.
- Moved ifd_tag_entry types to separate file.
- tags_unknown: Unknown-type tags can now satisfy the `UnknownTagValue`
interface to return `[]byte`.
- `IfdTagEntry.ValueBytes` automatically uses this. Else, error if not
`[]byte` or `string`.
- ifd_enumerate
- Now embed `ByteOrder` in `Ifd` (we otherwise require it to be passed
a bit too often).
- Renamed some occurences of "IFD" to "IB" where an actual IB in order
to not be confusing.
- Added some assertions and constants. Renamed from variable so it'd be easy to
tell if the byte-arrays, even though they come directly from the EXIF, which
specific offsets they start at (so no remove for confusion during enumeration
and building).
