Source code to reproduce miniboxed stream numbers

The only significant change is the addition of the Numeric trait
that is required for obtaining the best performance (it cuts down
the benchmark time by 50-100%). The rest is just the same source
code, modulo some whitespace my editor f'd up.

The Numeric issue is tracked here:
https://github.com/miniboxing/miniboxing-plugin/issues/154

And the numbers I got were:

```
[info] Benchmark                                        Mode  Samples    Score    Error  Units
[info] b.StreamsBenchmarks.baseline_cart                avgt        5    5.202 ±  0.544  ms/op
[info] b.StreamsBenchmarks.baseline_ref                 avgt        5   29.927 ±  6.279  ms/op
[info] b.StreamsBenchmarks.baseline_sum                 avgt        5    6.628 ±  2.986  ms/op
[info] b.StreamsBenchmarks.baseline_sumOfSquares        avgt        5    7.164 ±  1.254  ms/op
[info] b.StreamsBenchmarks.baseline_sumOfSquaresEven    avgt        5   12.746 ±  1.243  ms/op
[info] b.StreamsBenchmarks.streams_cart                 avgt        5   55.321 ± 12.771  ms/op
[info] b.StreamsBenchmarks.streams_ref                  avgt        5   63.508 ± 12.324  ms/op
[info] b.StreamsBenchmarks.streams_sum                  avgt        5   12.017 ±  0.879  ms/op
[info] b.StreamsBenchmarks.streams_sumOfSquares         avgt        5   12.977 ±  0.670  ms/op
[info] b.StreamsBenchmarks.streams_sumOfSquaresEven     avgt        5   52.159 ± 32.626  ms/op
[info] b.StreamsBenchmarks.views_cart                   avgt        5  314.893 ± 64.045  ms/op
[info] b.StreamsBenchmarks.views_ref                    avgt        5  157.288 ± 15.642  ms/op
[info] b.StreamsBenchmarks.views_sum                    avgt        5   83.296 ± 19.200  ms/op
[info] b.StreamsBenchmarks.views_sumOfSquares           avgt        5  200.816 ± 75.311  ms/op
[info] b.StreamsBenchmarks.views_sumOfSquaresEven       avgt        5  221.260 ± 13.161  ms/op
```

Review by @biboudis, hope you like it :)