Add nested thread pool support and use it in hyperparameter tuning

[mdekstrand]

This adds the plumbing support for multiple accelerator pools, runs accelerator tasks in separate pools when enabled, and uses nested pools in the multi-threaded Optuna tuner.

Closes #1124.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.53%. Comparing base (9e2604d) to head (2e97bc2).

Files with missing lines	Patch %	Lines
src/accel/tasks/mod.rs	55.00%	9 Missing ⚠️
src/accel/parallel.rs	78.94%	4 Missing ⚠️
src/lenskit/parallel/_pool.py	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1125      +/-   ##
==========================================
- Coverage   89.56%   89.53%   -0.04%     
==========================================
  Files         255      256       +1     
  Lines       17090    17167      +77     
==========================================
+ Hits        15307    15370      +63     
- Misses       1783     1797      +14     

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