Seek to independent IFD sequence in decoding

[197g]

This adds two capabilities to the encoder:

• next_directory to decode a directory without already decoding its contained image tags. It only reads a single contiguous range of file data and is thus quite a lot cheaper if only very specific tag values are of interest (or the image is to be skipped).
• current_directory_as_image to manually augment the IFD with its contained image.
• restart_at_{directory,image} to setup a new root directory. This can also be used to iterate SubIFDs in arbitrary depth by combining it with next_directory and appropriate tag reads.