Add GIF support

[ok-nick]

Changes in this pull request

Adds GIF support.

Related Issues

• Completes #476
• Partially completes #394

WIP Priorities

• [x] Parse entire GIF past pre-image-data block extensions
  • [x] Enables box hashing
  • [x] Completes XMP support (need to search pre and post block extensions)
• [x] Block extension parser should be incremental (via iterator)
  • [x] It should also iterate blocks past pre-image-data as described above
• [x] ~~Data hash location for XMP block~~
• [x] Add unit tests
  • [x] Composed manifest refs
  • [x] ~~Asset patches~~
  • [x] Data hash locations
  • [x] Box hash locations
  • [x] Remote refs

Checklist

• [x] This PR represents a single feature, fix, or change.
• [x] All applicable changes have been documented.
• [ ] Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[mauricefisher64]

Looks good. Just address the build issues and I can approve.

[ok-nick]

@mauricefisher64 Build issue is caused by an update in nightly toolchain, fixed in #498