Setup codecov report in CI

[pseudomuto]

Adding codecov to keep us honest

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a5d04a2). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #101   +/-   ##
=========================================
  Coverage          ?   97.31%           
=========================================
  Files             ?       23           
  Lines             ?      335           
  Branches          ?        0           
=========================================
  Hits              ?      326           
  Misses            ?        9           
  Partials          ?        0

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