more precise timer summary

[banteg]

• use time.perf_counter as the most precise way of measuring performance
• don't lose fast timer samples when calculating summary, use a list instead of a set

[codecov]

Codecov Report

Merging #113 (e12e34f) into main (5efff5f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #113   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          522       522           
  Branches        86        86           
=========================================
  Hits           522       522           

Impacted Files	Coverage Δ
devtools/timer.py	100.00% <100.00%> (ø)

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[samuelcolvin]

👍 thanks so much.