Fixes for problems found with 5.0.0-rc1

[jmalkin]

3 main changes:

• Fix dual C++/Python object handling in tuple and density to avoid leaking objects
• Ensure __str__ method always calls default to_string() and cannot accept parameters
• Add explicit tests of to_string() and __str__ to all tests
• NOMINSIZE in CMakeLists.txt to override Nanobind's -Os compiler flag

I'm not sure why a regular call to &sketch_type::to_string sometimes fails. Maybe I need to cast the the right type? But in a few places I used a lambda when it dind't seem necessary just because it worked.

If this looks good, I'll cherry-pick it into the 5.0.x branch.

[jmalkin]

The ref changes are based on the documentation here: https://nanobind.readthedocs.io/en/latest/ownership_adv.html#intrusive-reference-counting