Tests for firstindex and lastindex

[jeremypparker]

Tests for #84 to move things along

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.19%. Comparing base (dd6c877) to head (6efc354).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   84.15%   84.19%   +0.03%     
==========================================
  Files          15       15              
  Lines         871      873       +2     
==========================================
+ Hits          733      735       +2     
  Misses        138      138              

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[kalmarek]

@blegat for now ignore the failing pre; merge at will