[WIP] Generalize block sparse matricize

[mtfishman]

This rewrite block sparse matricize/unmatricize to call matricize/unmatricize blockwise, while before it called out to blockreshape. The previous logic was too limited when the blocks themselves have more structure and we don't want to simply reshape them, for example for Kronecker blocks.

Note that this is work in progress and will be coupled with a PR to KroneckerArrays.jl that will make use of this new design.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.50%. Comparing base (83ed436) to head (5944fb6).

Files with missing lines	Patch %	Lines
...rProductsExt/BlockSparseArraysTensorProductsExt.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
- Coverage   75.95%   75.50%   -0.46%     
==========================================
  Files          36       36              
  Lines        2059     2078      +19     
==========================================
+ Hits         1564     1569       +5     
- Misses        495      509      +14     

Flag	Coverage Δ
docs	7.61% <12.12%> (+0.36%)	⬆️

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