Support Set, keys, and values

[tkf]

No description provided.

[codecov-io]

Codecov Report

Merging #26 into master will decrease coverage by 1.26%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage   92.79%   91.52%   -1.27%     
==========================================
  Files           3        3              
  Lines         111      118       +7     
==========================================
+ Hits          103      108       +5     
- Misses          8       10       +2     

Impacted Files	Coverage Δ
src/SplittablesBase.jl	100.00% <ø> (ø)
src/implementations.jl	88.09% <66.66%> (-1.52%)	⬇️

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