This both abstracts the comparison, making it simpler to replace with
another data structure, and improves performance a little bit (by not
allocating a new comparison object each time).

```
Comparison:
                            empty
 using FULL_SET_DATA:  18144272.7 i/s
             v0.5.12:   7299127.9 i/s - 2.49x  slower
0.6.0.dev.g497b6c7c7:   6887110.4 i/s - 2.63x  slower

                             full
 using FULL_SET_DATA:   1842405.4 i/s
             v0.5.12:   1733075.9 i/s - 1.06x  slower
0.6.0.dev.g497b6c7c7:   1624744.9 i/s - 1.13x  slower

                      random data
 using FULL_SET_DATA:   7554952.4 i/s
0.6.0.dev.g497b6c7c7:   4376702.4 i/s - 1.73x  slower
             v0.5.12:   4361266.1 i/s - 1.73x  slower
```