Add addmul and submul to C and C++ interface

[saraedum]

Fixes #209.

Checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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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