SimpleCounter iterators are in insertion order

[nh13]

No description provided.

[nh13]

@tfenne Happy to add a unit test if you agree.

[codecov-commenter]

Codecov Report

Merging #73 (b862308) into master (b851fd3) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files          27       27           
  Lines         857      857           
  Branches       57       57           
=======================================
  Hits          799      799           
  Misses         58       58           

Flag	Coverage Δ
unittests	93.23% <100.00%> (ø)

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Impacted Files	Coverage Δ
...m/fulcrumgenomics/commons/util/SimpleCounter.scala	100.00% <100.00%> (ø)

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[tfenne]

@nh13 I think this is nice (at least gives some deterministic order) but just would want to make sure it's not massively slower to use a LinkedHashMap - hopefully once the value is inserted the overhead is zero?

Also, I can haz a def keys: Iterator[T]?