fast mapreduce for specific operators

[putianyi889]

• Support specific mapreduce #301

There are some type inconsistencies with the Base (when the eltype is short integer), otherwise there will need more work to make it work with InfiniteArrays.jl (see also #293). My suggestion is that since InfiniteArrays depends on FillArrays, the latter should not consider infinite cases.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (6f61dc3) to head (08f3338).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
- Coverage   99.91%   99.82%   -0.09%     
==========================================
  Files           8        8              
  Lines        1147     1153       +6     
==========================================
+ Hits         1146     1151       +5     
- Misses          1        2       +1     

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