Added HashSet

[VictorKoenders]

Closes #513

[codecov]

Codecov Report

Merging #516 (bbba17c) into trunk (108e03b) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            trunk     #516      +/-   ##
==========================================
+ Coverage   70.90%   71.08%   +0.17%     
==========================================
  Files          47       47              
  Lines        3159     3178      +19     
==========================================
+ Hits         2240     2259      +19     
  Misses        919      919              

Impacted Files	Coverage Δ
src/lib.rs	68.75% <ø> (ø)
src/features/impl_std.rs	80.60% <100.00%> (+1.79%)	⬆️
tests/alloc.rs	98.36% <100.00%> (+0.02%)	⬆️
tests/std.rs	100.00% <100.00%> (ø)

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