feat: code to help measure performance

[erdos]

[codecov[bot]]

Codecov Report

Merging #112 (be9f9dd) into master (27335c3) will decrease coverage by 0.85%. The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   93.10%   92.25%   -0.86%     
==========================================
  Files          27       28       +1     
  Lines        1872     1898      +26     
  Branches      104      105       +1     
==========================================
+ Hits         1743     1751       +8     
- Misses         25       42      +17     
- Partials      104      105       +1     

Impacted Files	Coverage Δ
src/stencil/perf.clj	30.76% <30.76%> (ø)

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