kron for RectDiagonal fill

[jishnub]

After this,

julia> E = FillArrays.RectDiagonal(Fill(4,300), (300,300));

julia> @btime kron($E, $E);
  1.040 ms (12 allocations: 2.07 MiB)

This runs out of memory on master as it materializes the result.

This PR also optimizes the following:

julia> D = Diagonal(Fill(10, 1000));

julia> @btime sparse($D);
  3.439 μs (3 allocations: 23.88 KiB) # master
  2.250 μs (3 allocations: 23.88 KiB) # PR

[codecov]

Codecov Report

Merging #272 (1491582) into master (e3c8eb9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #272   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files           5        5           
  Lines         836      844    +8     
=======================================
+ Hits          834      842    +8     
  Misses          2        2           

Impacted Files	Coverage Δ
src/FillArrays.jl	99.45% <100.00%> (+0.01%)	⬆️
src/fillalgebra.jl	100.00% <100.00%> (ø)

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