Specialize checkindex for an AbstractFill index

[jishnub]

checkindex for an AbstractFill index was being pirated by IniniteArrays.lj, and incompletely, since the Bool array behaves differently from an Int one. This PR moves the method to this package, where it belongs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.48%. Comparing base (877375f) to head (d8e3f3c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #411   +/-   ##
=======================================
  Coverage   99.48%   99.48%           
=======================================
  Files           8        8           
  Lines        1157     1159    +2     
=======================================
+ Hits         1151     1153    +2     
  Misses          6        6           

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[jishnub]

The test failures in InfiniteArrays.jl and InfiniteLinearAlgebra.jl are expected because of this type-piracy. Setting persistent_tasks=false in the Aqua checks gets the tests to pass.