add HMap ++

[alexeygorobets]

close #40

[codecov-io]

Codecov Report

Merging #42 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   82.62%   82.66%   +0.04%     
==========================================
  Files          13       13              
  Lines         374      375       +1     
  Branches       21       20       -1     
==========================================
+ Hits          309      310       +1     
  Misses         65       65

Impacted Files	Coverage Δ
core/src/main/scala/com/stripe/dagon/HMap.scala	95.83% <100%> (+0.18%)	⬆️

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[johnynek]

Thank you!