Demo: Remove SparseArrays from sysimage & set up as weak dep of Statistics

[IanButterworth]

Reverts commit 237c8d3 to remove SparseArrays from sysimage

Uses #47695 to set up SparseArrays as a weak dependency of Statistics, via a glue package.

See JuliaStats/Statistics.jl@86454ce for the relatively simple and minimal change. All code was copied across to the glue package untouched, with the addition of some imports.

This PR

% ./julia --startup-file=no

julia> @time using Statistics
[ Info: Precompiling Statistics [10745b16-79ce-11e8-11f9-7d13ad32a3b2]
  0.574351 seconds (15.58 k allocations: 1.226 MiB)

julia> @time using SparseArrays
[ Info: Precompiling SparseArrays [2f01184e-e22b-5df5-ae63-d93ebab69eaf]
[ Info: Precompiling SparseArraysGlue [b66d789c-28ca-57c3-870e-2abdfb76150c]
 15.779174 seconds (7.06 M allocations: 981.552 MiB, 0.48% compilation time)

julia> 
% ./julia --startup-file=no

julia> @time using Statistics
  0.011658 seconds (13.76 k allocations: 1.030 MiB)

julia> @time using SparseArrays
  4.618848 seconds (7.05 M allocations: 980.989 MiB, 1.46% compilation time)

Master

% ./julia --startup-file=no

julia> @time using Statistics
[ Info: Precompiling Statistics [10745b16-79ce-11e8-11f9-7d13ad32a3b2]
  0.516886 seconds (6.90 k allocations: 685.641 KiB)

julia> 
% ./julia --startup-file=no

julia> @time using Statistics
  0.013421 seconds (13.87 k allocations: 1.043 MiB)

4.6 seconds to load SparseArrays & the glue package seems a bit high. Worth checking on other non-macos platforms

cc. @KristofferC

[IanButterworth]

On linux

julia> @time using Statistics
  0.008449 seconds (12.34 k allocations: 961.833 KiB)

julia> @time using SparseArrays
  2.505745 seconds (7.05 M allocations: 980.824 MiB, 1.23% compilation time)