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 26.48402% with 161 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.74%. Comparing base (394c629) to head (3e0efa2).
⚠️ Report is 78 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hamiltonian.rs	19.38%	79 Missing ⚠️
src/nuts.rs	23.28%	56 Missing ⚠️
src/transformed_hamiltonian.rs	0.00%	16 Missing ⚠️
src/euclidean_hamiltonian.rs	62.50%	6 Missing ⚠️
src/sampler.rs	66.66%	2 Missing ⚠️
src/stepsize/adapt.rs	50.00%	1 Missing ⚠️
src/transform_adapt_strategy.rs	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (394c629) and HEAD (3e0efa2). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (394c629)	HEAD (3e0efa2)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #36       +/-   ##
===========================================
- Coverage   83.77%   59.74%   -24.04%     
===========================================
  Files           8       27       +19     
  Lines        1923     6585     +4662     
===========================================
+ Hits         1611     3934     +2323     
- Misses        312     2651     +2339     

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