RFC: make FillArrays.jl a weakdep

[dkarrasch]

This is as far as I got. I got stuck at the MulAdd(::Mul) constructor, which calls mulzeros, which calls Zeros for Number eltypes. So, either this means we have a strong dependency, or there is a work-around. Perhaps, after https://github.com/JuliaArrays/FillArrays.jl/pull/286 FillArrays.jl is light enough anyway, but if there is a simple enough work-around, I think it would be still beneficial to reduce strong dependencies as much as possible. I'm leaving for summer holidays, so I'm putting this up for comments.

[codecov[bot]]

Codecov Report

Patch coverage: 72.72% and project coverage change: -0.05% :warning:

Comparison is base (14721f1) 87.08% compared to head (227b954) 87.04%. Report is 4 commits behind head on master.

:exclamation: Current head 227b954 differs from pull request most recent head 7bbaeb9. Consider uploading reports for the commit 7bbaeb9 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   87.08%   87.04%   -0.05%     
==========================================
  Files          10       11       +1     
  Lines        1773     1775       +2     
==========================================
+ Hits         1544     1545       +1     
- Misses        229      230       +1     

Files Changed	Coverage Δ
src/diagonal.jl	92.30% <ø> (+9.25%)	:arrow_up:
src/memorylayout.jl	84.85% <ø> (+0.32%)	:arrow_up:
src/muladd.jl	77.31% <ø> (+2.01%)	:arrow_up:
ext/ArrayLayoutsFillArraysExt.jl	70.49% <70.49%> (ø)
src/ArrayLayouts.jl	82.22% <100.00%> (-0.58%)	:arrow_down:
src/ldiv.jl	90.40% <100.00%> (+0.07%)	:arrow_up:
src/mul.jl	83.66% <100.00%> (-1.06%)	:arrow_down:

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[dkarrasch]

I pushed things a bit further. Controversial aspects are the above-mentionen mulzeros issue and the removal of + and - overloads for UniformScalings. FWIW, this now clearly shows the dependent parts, so comments are most welcome.