Bounds-checking in triangular indexing branches

[jishnub]

After this PR, calls like the following would throw:

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

julia> U[1,0]
0

Currently, there's no bounds-checking on the indices if they don't correspond to stored values.

Also, this PR checks for compatible types in assigning to the diagonal of unit triangular matrices. This fixes issues like, e.g.:

julia> A = @SMatrix [1 2; 3 4]
2×2 SMatrix{2, 2, Int64, 4} with indices SOneTo(2)×SOneTo(2):
 1  2
 3  4

julia> M = fill(A, 2, 2)
2×2 Matrix{SMatrix{2, 2, Int64, 4}}:
 [1 2; 3 4]  [1 2; 3 4]
 [1 2; 3 4]  [1 2; 3 4]

julia> U = UnitUpperTriangular(M)
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[1,1] = 1
1

After this,

julia> U[1,1] = 1
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type SMatrix{2, 2, Int64, 4}

However, this depends on JuliaLang/julia#58209 for the SizedArrays tests to pass, as oneunit currently doesn't work with a SizedArray.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.88%. Comparing base (cdd135e) to head (a36f65e).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1305   +/-   ##
=======================================
  Coverage   93.87%   93.88%           
=======================================
  Files          34       34           
  Lines       15857    15879   +22     
=======================================
+ Hits        14886    14908   +22     
  Misses        971      971           

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[dkarrasch]

This looks like ready to go, doesn't it? Should then be included in #1420.