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

Attention: Patch coverage is 35.16484% with 177 lines in your changes missing coverage. Please review.

Project coverage is 69.38%. Comparing base (394c629) to head (b2366c1).
Report is 63 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hamiltonian.rs	18.81%	82 Missing ⚠️
src/nuts.rs	23.28%	56 Missing ⚠️
src/transformed_hamiltonian.rs	0.00%	16 Missing ⚠️
src/stepsize_adapt.rs	68.18%	7 Missing ⚠️
src/euclidean_hamiltonian.rs	62.50%	6 Missing ⚠️
src/transform_adapt_strategy.rs	0.00%	6 Missing ⚠️
src/chain.rs	92.00%	2 Missing ⚠️
src/sampler.rs	66.66%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (394c629) and HEAD (b2366c1). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (394c629)	HEAD (b2366c1)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #36       +/-   ##
===========================================
- Coverage   83.77%   69.38%   -14.40%     
===========================================
  Files           8       18       +10     
  Lines        1923     4680     +2757     
===========================================
+ Hits         1611     3247     +1636     
- Misses        312     1433     +1121     

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