Use add! in unmatricizeadd!

[mtfishman]

This helps with contract in GradedArrays, since full broadcasting isn't working there right now so when iszero(β), add! has a simpler codepath. Also more generally it makes unmatricizeadd! (and therefore contract[add][!]) benefit from optimizations to add!.

Note that this PR exposed a bug in broadcasting empty StridedViews which I made a PR to fix here: QuantumKitHub/Strided.jl#50. For now I'm working around that issue in this PR but we can remove that workaround once it is fixed in Strided.jl.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.74%. Comparing base (a4d5381) to head (3074fa8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
- Coverage   71.91%   71.74%   -0.17%     
==========================================
  Files          24       24              
  Lines        1214     1214              
==========================================
- Hits          873      871       -2     
- Misses        341      343       +2     

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

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[mtfishman]

Test failures should be fixed by QuantumKitHub/Strided.jl#50 (see updated comment above).