Add addmul and submul to C and C++ interface

[saraedum]

Fixes #209.

Checklist

• Added an entry in doc/news/.
• Added a test/benchmark for this change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (53a9cf2) 92.23% compared to head (b8732e2) 92.23%.

❗ Current head b8732e2 differs from pull request most recent head a872d1e. Consider uploading reports for the commit a872d1e to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #258   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         105      105           
  Lines        2124     2124           
=======================================
  Hits         1959     1959           
  Misses        165      165           

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