Use indices instead of stride in default_blasmul

[jishnub]

There's no need to use linear indexing here, since the conversion from Cartesian to linear is cheap.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.51%. Comparing base (b9b424e) to head (49140ea).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
- Coverage   89.52%   89.51%   -0.02%     
==========================================
  Files          11       11              
  Lines        1938     1936       -2     
==========================================
- Hits         1735     1733       -2     
  Misses        203      203              

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[dlfivefifty]

How do you know it’s cheap?

[jishnub]

In most cases, accessing memory will be significantly more expensive than integer multiplication and addition. In any case, this change is unnecessary, so I'm reverting this in #260.

[dlfivefifty]

Please don't merge changes like this without review.