Fix * between matrix and vector with non-concrete element types

[odow]

Closes #336

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.56%. Comparing base (ce33a4b) to head (5f3ce65).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #337      +/-   ##
==========================================
+ Coverage   90.55%   90.56%   +0.01%     
==========================================
  Files          22       22              
  Lines        2265     2269       +4     
==========================================
+ Hits         2051     2055       +4     
  Misses        214      214              

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[blegat]

Good catch