This data structure will be used to implement concurrency in ΔBtail.

* t2+RangedMap: (45 commits)
  .
  .
  .
  X fixes for empty set/range
  .
  .
  .
  .
  X Tests for Get
  .
  X SetRange draftly works
  .
  .
  .
  .
  .
  .
  .
  X need to keep Value first so that sizeof(set-entry) = sizeof(KeyRange)
  .
  ...

see github.com/golang/go/issues/48651