Provide a way to retrieve all tags defined within a file

[gschulze]

Currently, it is only possible to retrieve single tags via get_tag(...) and find_tag(...). However, one might be interested in obtaining all defined tags, for instance, when modifying the TIFF and writing it to a file again. Two possible APIs come to my mind:

• fn get_all_tags(): impl<Iterator<Item = Tag>>, returning all tags defined in the TIFF file, or
• fn get_all_tags(): HashMap<Tag, ifd::Value>, returning all tags along with their value

It would be really nice to have this addition.

[spoutn1k]

I'm working on #242 to do just that, it is far from ready but I am implementing a newtype over your second proposition to streamline tag access. I can update here once I have a better prototype.