neo@20e8f37b

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

* t2+RangedMap: (45 commits)
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  X fixes for empty set/range
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  X Tests for Get
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  X SetRange draftly works
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  X need to keep Value first so that sizeof(set-entry) = sizeof(KeyRange)
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see github.com/golang/go/issues/48651