Specialize indexing with a AbstractFill{Bool} index

[jishnub]

This improves performance, e.g. in

julia> v = rand(1000);

julia> f = Fill(true, length(v));

julia> @btime $v[$f];
  1.350 μs (3 allocations: 7.88 KiB) # master
  629.480 ns (3 allocations: 7.88 KiB) # this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.62%. Comparing base (877375f) to head (07a1b62).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
- Coverage   99.48%   98.62%   -0.86%     
==========================================
  Files           8        8              
  Lines        1157     1162       +5     
==========================================
- Hits         1151     1146       -5     
- Misses          6       16      +10     

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