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
|--------------------:|--------------------:|--------:|----------:|:----------
|              325.60 |        3,071,225.77 |    4.8% |      0.77 | `skip list (point reads)`
|              297.15 |        3,365,278.10 |    1.7% |      0.72 | `skip list (prefix reads)`
|              408.79 |        2,446,222.23 |    1.0% |      1.03 | `skip list (range reads)`
|              261.88 |        3,818,471.08 |    1.3% |      0.73 | `skip list (point writes)`
|              253.54 |        3,944,191.08 |    0.1% |      0.61 | `skip list (prefix writes)`
|              258.73 |        3,865,078.52 |    0.8% |      0.62 | `skip list (range writes)`
|              489.56 |        2,042,648.19 |    1.8% |      0.01 | `skip list (monotonic increasing point writes)`
|               14.83 |       67,446,579.75 |    0.1% |      0.04 | `radix tree (point reads)`
|               59.68 |       16,756,917.37 |    0.1% |      0.14 | `radix tree (prefix reads)`
|              287.32 |        3,480,485.22 |    1.2% |      0.69 | `radix tree (range reads)`
|               46.59 |       21,461,855.59 |    0.2% |      0.12 | `radix tree (point writes)`
|               83.70 |       11,946,755.99 |    0.1% |      0.20 | `radix tree (prefix writes)`
|              100.75 |        9,925,723.26 |    0.6% |      0.25 | `radix tree (range writes)`
|              118.37 |        8,448,345.29 |    0.6% |      0.01 | `radix tree (monotonic increasing point writes)`