nmod_mat.jl: some more add!/sub!/mul! changes

[fingolfin]

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[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.95%. Comparing base (d12b72e) to head (3efa482).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/flint/nmod_mat.jl	85.71%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1981   +/-   ##
=======================================
  Coverage   87.94%   87.95%           
=======================================
  Files          98       98           
  Lines       36072    36068    -4     
=======================================
- Hits        31725    31724    -1     
+ Misses       4347     4344    -3     

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