feat: specify equations via extern C functions

[martinjrobins]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 34.48276% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.85%. Comparing base (917496d) to head (d58352a).

Files with missing lines	Patch %	Lines
diffsol/src/ode_equations/diffsl.rs	43.47%	13 Missing ⚠️
diffsol/src/ode_solver/builder.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
- Coverage   73.97%   73.85%   -0.13%     
==========================================
  Files          84       84              
  Lines        8881     8892      +11     
==========================================
- Hits         6570     6567       -3     
- Misses       2311     2325      +14     

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