Define checkindex with AbstractFill only for AbstractInfUnitRange

[jishnub]

This avoids type piracy in this package.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.66%. Comparing base (41cc9c5) to head (179eacd).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   89.45%   89.66%   +0.21%     
==========================================
  Files          10       10              
  Lines        1138     1142       +4     
==========================================
+ Hits         1018     1024       +6     
+ Misses        120      118       -2     

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[dlfivefifty]

can you add a unit test