Faster sparse for AbstractFill

[jishnub]

There aren't particularly useful, but it can't hurt to have some faster methods.

julia> F = Fill(3, 4_000);

julia> @b sparse(F)
11.541 μs (19 allocs: 197.219 KiB) # master
3.501 μs (7 allocs: 62.656 KiB) # This PR

julia> F = Fill(3, 1_000, 1_000);

julia> @b SparseMatrixCSC{Int32,Int32}(F)
5.623 ms (52 allocs: 17.417 MiB) # master
546.454 μs (19 allocs: 7.653 MiB) # This PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.48%. Comparing base (877375f) to head (5ad9b74).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #406   +/-   ##
=======================================
  Coverage   99.48%   99.48%           
=======================================
  Files           8        8           
  Lines        1157     1161    +4     
=======================================
+ Hits         1151     1155    +4     
  Misses          6        6           

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