Support saving JPEG comments

[radarhere]

Suggestions for python-pillow#6774

1. Save comments from any image format by default. At the moment, your code doesn't produce a comment when saving a GIF.

from PIL import Image
im = Image.open("Tests/images/multiple_comments.gif")
im.seek(1)
print(im.info.get("comment"))  # b'Test comment 1\nTest comment 2'

im.save("out.jpg")
print(Image.open("out.jpg").info.get("comment"))  # None

This PR allows that to happen.

2. Use _binary instead of struct. This isn't a correction to your code. It's actually taking advice from your code that _binary can be used instead of struct and applying that to another place in JpegImagePlugin.

3. Test that comment is reread. Your test currently saves info["comment"] and then checks that it appears in app["COM"]. It seems more logical to me to check that it appears in info["comment"].

[smason]

Those all look sensible, thanks!

I wasn't sure about where to pull any prior comment from; im.info['comment'] looks much better.

I also wasn't sure about whether it was appropriate to clean up other parts of the code base, e.g. your struct vs. _binary change. It seemed like there were going to be quite a few, so I was going to open another pull request for that when I'd got a few together.