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
BenchmarkQuery                          691           97.5          -85.89%
BenchmarkQuerySmallEpsilon              44491         6500          -85.39%
BenchmarkInsertBiasedSmallEpsilon       2641          914           -65.39%
BenchmarkInsertTargetedSmallEpsilon     1016          442           -56.50%
BenchmarkInsertBiased                   324           172           -46.91%
BenchmarkInsertTargeted                 294           171           -41.84%
```