Add a dedicated Directory type

[197g]

The key motivation here is the ability to represent the tags within a
directory separate from a file. This allows us to represent directories
that are not image directories, i.e. the EXIF directory, the related GPS
directory, and other sub directories. These are defined by a 'pointer'
value in another tag, an offset into the file. They can then be read
from that immediate location.

Another PR will add methods to turn the entries of a Directory into ifd::Values by using the reader / bigtiff / byte order / limits configuration from a value, which would then be enough to make Exif and GPS directories useful.

[197g]

Is this repository intentionally setup to use squash merges? That would be different from all the rest of the image-rs repos.

[fintelia]

I probably switched it the last time I made changes to this repository's settings. If you'd rather other merge types (or even just want to use them occasionally) feel free to re-enable them in the settings.

I have a slight preference for squash merges since PRs frequently have a lot of pointless "fix typo" style commits (mine certainly do!) and GitHub's commit history page doesn't let you filter to only merges. By contrast, if you do want to see the individual commits for a merged PR, GitHub saves them even for deleted branches