Add findmax, findmin, argmax, and argmin

[tkf]

No description provided.

[codecov]

Codecov Report

Merging #42 (941ae69) into master (7229305) will increase coverage by 0.08%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   82.16%   82.25%   +0.08%     
==========================================
  Files           7        7              
  Lines         342      355      +13     
==========================================
+ Hits          281      292      +11     
- Misses         61       63       +2     

Flag	Coverage Δ
unittests	82.25% <84.61%> (+0.08%)	⬆️

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Impacted Files	Coverage Δ
src/Folds.jl	100.00% <ø> (ø)
src/testing.jl	88.61% <ø> (ø)
src/utils.jl	44.68% <75.00%> (+2.82%)	⬆️
src/reduce.jl	93.04% <88.88%> (-0.36%)	⬇️

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