Fix scaling unit triangular matrices

[jishnub]

After this, the following works:

julia> U = UnitUpperTriangular(fill(SMatrix{2,2}([1 2; 3 4]), 2, 2))
2×2 UnitUpperTriangular{SMatrix{2, 2, Int64, 4}, Matrix{SMatrix{2, 2, Int64, 4}}}:
 [1 0; 0 1]  [1 2; 3 4]
     ⋅       [1 0; 0 1]

julia> U * 2
2×2 UpperTriangular{SMatrix{2, 2, Int64, 4}, Matrix{SMatrix{2, 2, Int64, 4}}}:
 [2 0; 0 2]  [2 4; 6 8]
     ⋅       [2 0; 0 2]

julia> U = UnitUpperTriangular(@SMatrix ones(4,4))
4×4 UnitUpperTriangular{Float64, SMatrix{4, 4, Float64, 16}} with indices SOneTo(4)×SOneTo(4):
 1.0  1.0  1.0  1.0
  ⋅   1.0  1.0  1.0
  ⋅    ⋅   1.0  1.0
  ⋅    ⋅    ⋅   1.0

julia> U * 2
4×4 UpperTriangular{Float64, MMatrix{4, 4, Float64, 16}} with indices SOneTo(4)×SOneTo(4):
 2.0  2.0  2.0  2.0
  ⋅   2.0  2.0  2.0
  ⋅    ⋅   2.0  2.0
  ⋅    ⋅    ⋅   2.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.77%. Comparing base (ea8e858) to head (2148fa4).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1346      +/-   ##
==========================================
+ Coverage   93.75%   93.77%   +0.01%     
==========================================
  Files          34       34              
  Lines       15761    15752       -9     
==========================================
- Hits        14777    14771       -6     
+ Misses        984      981       -3     

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