Add cross-compilation example for STM32F303

[sunsided]

This reworks how the no_std / std feature work, introduces explicit libm support for and adds an STM32F303 embedded example that strictly requires no_std and no allocations. The example builds both floating-point and fixed-point support.

[codecov]

Codecov Report

Attention: Patch coverage is 84.55882% with 42 lines in your changes missing coverage. Please review.

Project coverage is 87.63%. Comparing base (018e40f) to head (8047206).

Files	Patch %	Lines
src/measurement.rs	67.74%	20 Missing ⚠️
src/kalman.rs	68.42%	18 Missing ⚠️
src/matrix.rs	96.46%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   86.96%   87.63%   +0.66%     
==========================================
  Files           5        5              
  Lines        1412     1488      +76     
==========================================
+ Hits         1228     1304      +76     
  Misses        184      184              

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