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

Attention: Patch coverage is 93.22034% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.72%. Comparing base (95703b5) to head (4dde1de).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/triangular.jl	93.10%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1305      +/-   ##
==========================================
- Coverage   93.73%   93.72%   -0.02%     
==========================================
  Files          34       34              
  Lines       15743    15781      +38     
==========================================
+ Hits        14757    14791      +34     
- Misses        986      990       +4     

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