Optimize Implementation

[achille-roussel]

Hello,

This PR modifies the internal implementation to reduce the CPU and memory footprint of the algorithms, here are numbers to compare upstream with this change on the benchmark that I added:

benchmark                  old ns/op     new ns/op     delta
BenchmarkNaturalBreaks     292883        143310        -51.07%

benchmark                  old allocs     new allocs     delta
BenchmarkNaturalBreaks     355            4              -98.87%

benchmark                  old bytes     new bytes     delta
BenchmarkNaturalBreaks     49840         28784         -42.25%

[coveralls]

Pull Request Test Coverage Report for Build 21

• 146 of 152 (96.05%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.3%) to 97.727%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
jenks.go	146	152	96.05%

Totals
Change from base Build 12:	-2.3%
Covered Lines:	258
Relevant Lines:	264

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💛 - Coveralls

[YetAnotherMatt]

Hi @achille-roussel I can only apologise for ignoring your PR for so long (and I see you've understandably forged ahead with your own fork). I had notifications turned off, and have only just realised that this PR existed.