Quantile: significant performance improvements

I started by adding more benchmarks to hit a variety of scenarios:

* Both targeted and biased streams
* Smaller epsilon (for a larger internal sample list -- important because one of the improvements was to remove `container/list`)
* Querying

Then I made three different changes. Each of those commits includes intermediate benchmark output.

Here's the final result:

```
benchmark                              old ns/op    new ns/op    delta
BenchmarkInsertTargeted                      297          173  -41.75%
BenchmarkInsertTargetedSmallEpsilon         1029          445  -56.75%
BenchmarkInsertBiased                        325          174  -46.46%
BenchmarkInsertBiasedSmallEpsilon           2642          923  -65.06%
BenchmarkQuery                               700          110  -84.29%
BenchmarkQuerySmallEpsilon                 44309         8609  -80.57%
```