Release v0.20.0

Additions:
- queue: add Push and PopLast methods

Changes:
- queue: allow Peek to accept negative offsets

- mdiff: rework how formatting is done
  The Diff and Patch now have Format methods.
  The formatting functions have been renamed, and take chunks.

- mlink,ring: split ring into its own package
  Use ring.New / ring.Of instead of mlink.NewRing / mlink.RingOf.