A tag tree is a data structure used for deserializing JPEG2000
packet headers.
We don't use them for anything yet, except from tests.
The implementation feels a bit awkward to me, but we can always polish
it later.
The spec thankfully includes two concrete examples. The code is
correct enough to pass those -- I added them as test.
JPEG2000 is the last image format used in PDF filters that we
don't have a loader for. Let's change that.
This adds all the scaffolding, but no actual implementation yet.
