use unwrapping mechanism for triangular matrices

[dkarrasch]

This is to make use of JuliaLang/julia#50058 after that one is merged.

[codecov]

Codecov Report

Merging #396 (65cb367) into main (0eb9c04) will decrease coverage by 1.53%.
The diff coverage is 64.14%.

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
- Coverage   93.84%   92.31%   -1.53%     
==========================================
  Files          12       12              
  Lines        7521     7600      +79     
==========================================
- Hits         7058     7016      -42     
- Misses        463      584     +121     

Impacted Files	Coverage Δ
src/linalg.jl	86.52% <63.90%> (-9.58%)	⬇️
src/sparsevector.jl	95.50% <100.00%> (ø)

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[dkarrasch]

Finally, finally! This not only contains modifications due to the unwrapping mechanism, but also a gentle rewiring of type unions. This was carefully tested with profiling in view of short load times.

[dkarrasch]

Ok, shall we merge? After this we should directly bump the stdlib, so I can do a little clean-up of intermediate left-overs, and then we should backport all of this as fast as possible to the alpha-release of v1.10.