Rework the matricize interface

[mtfishman]

Tweak the new matricize interface introduced in #101 and #103 a bit. This makes the overloading syntax more verbose for trivial_axis and tensor_product_axis, but has a clear logical flow so that less verbose versions all call down to those more verbose versions.

[codecov]

Codecov Report

❌ Patch coverage is 61.11111% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.88%. Comparing base (151f271) to head (134bd49).

Files with missing lines	Patch %	Lines
src/matricize.jl	41.66%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   89.66%   87.88%   -1.78%     
==========================================
  Files          17       17              
  Lines         629      644      +15     
==========================================
+ Hits          564      566       +2     
- Misses         65       78      +13     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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