WIP Add walnuts implementation

[aseyboldt]

Completely untested and probably broken at the moment!
Also needs some cleanup, right now everything new is in one big function.

Reference implementation: https://github.com/bob-carpenter/walnuts
paper of the algorithm: https://arxiv.org/pdf/2506.18746

[codecov-commenter]

Codecov Report

❌ Patch coverage is 19.11765% with 165 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.81%. Comparing base (394c629) to head (ee9f927).
⚠️ Report is 117 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dynamics/hamiltonian.rs	0.00%	98 Missing ⚠️
src/nuts.rs	22.66%	58 Missing ⚠️
src/dynamics/transformed_hamiltonian.rs	66.66%	5 Missing ⚠️
src/sampler.rs	66.66%	2 Missing ⚠️
src/external_adapt_strategy.rs	0.00%	1 Missing ⚠️
src/stepsize/adapt.rs	50.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (394c629) and HEAD (ee9f927). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (394c629)	HEAD (ee9f927)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #36       +/-   ##
===========================================
- Coverage   83.77%   59.81%   -23.97%     
===========================================
  Files           8       31       +23     
  Lines        1923     8194     +6271     
===========================================
+ Hits         1611     4901     +3290     
- Misses        312     3293     +2981     

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