feat:metadata expose MetadataCursor and add accessors for ImageFileDirectoryReader

[feefladder]

#82 gives the user control over metadata reading, and some methods of MetadataCursor were public, but not the type itself. This allows the user to read tag type, count and value by themselves if they want to do complex metadata caching.

[kylebarron]

Thanks; I need to think about this though. There's a proliferation of public APIs and I'd much rather figure out how to remove public APIs than add new ones.

[feefladder]

I understand. there's two pathways I see here:

1. minimize methods on MetadataCursor (new(),with_offset,new_with_offset,seek)
2. Don't expose MetadataCursor at all.
   • add a ImageFileDirectoryReader.peek_tag(&self, tag_offset: u64) -> (Tag, TagType, count: u64, Option<offset: u64>) function.
   • This could stand on its own, or (Optional) refactor the read_tag,read_tag_value functions:
     1. peek_tag, if Some(offset) -> move cursor
     2. read tag value from cursor
        But I think the refactor loses the scope of this PR and would fit a separate PR (if at all).
   • I've made a sample implementation here for the peek_tag function.