Implement ImmutableHashSet<T>

[sharwell]

Fixes #39

[codecov]

Codecov Report

Merging #50 into master will increase coverage by 0.79%.
The diff coverage is 88.37%.

@@             Coverage Diff              @@
##             master      #50      +/-   ##
============================================
+ Coverage     81.17%   81.97%   +0.79%     
- Complexity     1167     1325     +158     
============================================
  Files            40       44       +4     
  Lines          6938     7693     +755     
  Branches        877      965      +88     
============================================
+ Hits           5632     6306     +674     
- Misses         1011     1073      +62     
- Partials        295      314      +19

Impacted Files	Coverage Δ
src/com/tvl/util/StrongBox.java	100% <100%> (ø)
test/com/tvl/util/ImmutableSetTest.java	98.4% <100%> (+1.27%)	⬆️
src/com/tvl/util/Immutables.java	83.33% <57.14%> (-10.79%)	⬇️
test/com/tvl/util/ImmutableHashSetBuilderTest.java	87.57% <87.57%> (ø)
src/com/tvl/util/ImmutableHashSet.java	88.05% <88.05%> (ø)
test/com/tvl/util/ImmutableHashSetTest.java	93.25% <93.25%> (ø)
... and 4 more

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