Specialize isassigned for AbstractFill

[jishnub]

Since an AbstractFill is by definition filled with values, we may short-circuit isassigned:

julia> F = Fill(3, 100, 100);

julia> @btime isassigned($(Ref(F))[], 1, 1)
  24.297 ns (0 allocations: 0 bytes) # master
  3.360 ns (0 allocations: 0 bytes) # PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (99278fa) to head (e36922a).
Report is 16 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #351   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files           8        8           
  Lines        1037     1061   +24     
=======================================
+ Hits         1036     1060   +24     
  Misses          1        1           

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[longemen3000]

seems good to merge, at first i thought that this would affect OneElement, but that type subtypes AbstractArray instead

[jishnub]

I've specialized isassigned for a OneElement as well, as the same logic would apply. Since the array is always initialized with a value, the elements should all be assigned as long as they are in range.