Implement MathsProvider pattern for STL and XSIMD backends

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (b8dc376) 95.65% compared to head (4a3e424) 95.65%.

Files	Patch %	Lines
RTNeural/common.h	62.50%	3 Missing :warning:
RTNeural/activation/activation.h	93.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #115 +/- ## ========================================== - Coverage 95.65% 95.65% -0.01% ========================================== Files 55 57 +2 Lines 3940 3866 -74 ========================================== - Hits 3769 3698 -71 + Misses 171 168 -3 ```

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