Reduce memory footprint.

[jm-franc]

Divides the memory footprint of a sampler instance by 7 (from ~0.5MB to 65KB), at the expense of a small increase in over-sampling chances.

https://github.com/uber-go/zap/issues/1516

[jm-franc]

/assign sywhang

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 98.70%. Comparing base (07077a6) to head (59499db). :warning: Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1517      +/-   ##
==========================================
+ Coverage   98.59%   98.70%   +0.11%     
==========================================
  Files          53       53              
  Lines        3565     3023     -542     
==========================================
- Hits         3515     2984     -531     
+ Misses         42       31      -11     
  Partials        8        8              

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.