Fix matrix multiplication with non-commutative elements

[jishnub]

Fixes various issues like

julia> using FillArrays, LinearAlgebra, StaticArrays

julia> S = SMatrix{2,3}(1:6);

julia> A = reshape([S,2S,3S,4S],2,2);

julia> F = Fill(S',2,2);

julia> A * F
2×2 Matrix{SMatrix{2, 2, Int64, 4}}:
 [140 176; 176 224]  [140 176; 176 224]
 [210 264; 264 336]  [210 264; 264 336]

julia> mul!(similar(A * F), A, F) ≈ A * F
true

This example errors on master currently as the order of terms is incorrect (assumes commutativity).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (373183f) 99.89% compared to head (01b083a) 99.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   99.89%   99.89%   -0.01%     
==========================================
  Files           8        8              
  Lines         919      916       -3     
==========================================
- Hits          918      915       -3     
  Misses          1        1              

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[jishnub]

This should be ready now

[jishnub]

The coverage reduction seems spurious