optimize Chain.Ops()

[mmcloughlin]

Use linear 2SUM-like algorithm in the typical case where the chain is
ascending.

[codecov-commenter]

Codecov Report

Merging #99 (48ef593) into master (c4e8a5d) will increase coverage by 0.89%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   70.80%   71.69%   +0.89%     
==========================================
  Files          29       29              
  Lines        1151     1166      +15     
==========================================
+ Hits          815      836      +21     
+ Misses        276      270       -6     
  Partials       60       60              

Flag	Coverage Δ
unittests	71.69% <100.00%> (+0.89%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
chain.go	46.87% <100.00%> (+28.50%)	⬆️

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