Implement a HashEdgeMap<T> offering lock-free O(1) lookups

[codecov-io]

Codecov Report

Merging #28 into master will decrease coverage by 0.26%. The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   61.91%   61.64%   -0.27%     
==========================================
  Files         371      372       +1     
  Lines       18386    18483      +97     
  Branches     3499     3515      +16     
==========================================
+ Hits        11383    11394      +11     
- Misses       5747     5832      +85     
- Partials     1256     1257       +1

Impacted Files	Coverage Δ
...rc/org/antlr/v4/runtime/atn/LexerATNSimulator.java	87.39% <ø> (ø)	:arrow_up:
...va/src/org/antlr/v4/runtime/dfa/SparseEdgeMap.java	0% <0%> (-68%)	:arrow_down:
...src/org/antlr/v4/runtime/dfa/SingletonEdgeMap.java	28.57% <100%> (-2.05%)	:arrow_down:
...Java/src/org/antlr/v4/runtime/dfa/HashEdgeMap.java	74.71% <74.71%> (ø)
...ava/src/org/antlr/v4/runtime/dfa/ArrayEdgeMap.java	29.41% <81.81%> (-6.59%)	:arrow_down:

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