A data structure for optimistic concurrency control on ranges of bitwise-lexicographically-ordered keys.

Intended to replace FoundationDB's skip list.

# FoundationDB's benchmark

## Skip list

```
New conflict set: 4.189 sec
                  0.298 Mtransactions/sec
                  1.194 Mkeys/sec
Detect only:      3.990 sec
                  0.313 Mtransactions/sec
                  1.253 Mkeys/sec
Skiplist only:    2.849 sec
                  0.439 Mtransactions/sec
                  1.755 Mkeys/sec
Performance counters:
               Build: 0.0913
                 Add: 0.0998
              Detect: 3.99
              D.Sort: 0.808
           D.Combine: 0.0309
         D.CheckRead: 1.67
   D.CheckIntraBatch: 0.0305
        D.MergeWrite: 1.18
      D.RemoveBefore: 0.265
```

## Radix tree (this implementation)

```
New conflict set: 2.965 sec
                  0.422 Mtransactions/sec
                  1.686 Mkeys/sec
Detect only:      2.761 sec
                  0.453 Mtransactions/sec
                  1.811 Mkeys/sec
Skiplist only:    1.580 sec
                  0.791 Mtransactions/sec
                  3.165 Mkeys/sec
Performance counters:
               Build: 0.0902
                 Add: 0.107
              Detect: 2.76
              D.Sort: 0.809
           D.Combine: 0.0309
         D.CheckRead: 0.658
   D.CheckIntraBatch: 0.0294
        D.MergeWrite: 0.921
      D.RemoveBefore: 0.305
```

# Our benchmark

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|              263.10 |        3,800,889.33 |    1.6% |      0.64 | `skip list (point reads)`
|              256.36 |        3,900,800.96 |    1.7% |      0.61 | `skip list (prefix reads)`
|              526.95 |        1,897,714.85 |    1.6% |      1.23 | `skip list (range reads)`
|              546.54 |        1,829,676.97 |    0.5% |      1.31 | `skip list (point writes)`
|              539.23 |        1,854,511.73 |    0.5% |      1.29 | `skip list (prefix writes)`
|              266.68 |        3,749,799.99 |    0.6% |      0.65 | `skip list (range writes)`
|               18.31 |       54,612,706.60 |    0.4% |      0.04 | `radix tree (point reads)`
|               48.78 |       20,498,870.77 |    0.2% |      0.12 | `radix tree (prefix reads)`
|              347.16 |        2,880,474.86 |    0.9% |      0.83 | `radix tree (range reads)`
|               34.80 |       28,734,706.67 |    3.6% |      0.09 | `radix tree (point writes)`
|               70.34 |       14,216,970.16 |    1.4% |      0.17 | `radix tree (prefix writes)`
|               82.41 |       12,134,555.86 |    1.0% |      0.20 | `radix tree (range writes)`