Tidy-up to-dos

• jaxtyping
• remove jax.dtypes.canonicalize_dtype
• sympy2jax
• ruff
• private modules
• import foo as foo
• abstract vars (+no more init=False)
• pyproject.toml: no setup.py; no .isort.cfg, no .flake8; use hatchling; no MANIFEST, update release action to v2
• Have diffrax.RESULTS return a wrapper around integers, to prevent comparisons like sol.result == 0
• use converter and __check_init__ instead of __post_init__, e.g. for SubSaveAt.
• make sure default arguments don't have conditionals, for the sake of IDE integration
• typing -> collections.abc
• remove AbstractStepSizeController.wrap_solver.
• handle rtol and atol defaults.
• linear and nonlinear solves (including switching to Optx norms)
• shaped_allclose -> tree_equal
• ruff-format
• remove _ad.py
• jrandom -> jr
• pyright
• switch path's evaluate(self, t0, t1) -> __call__(self, s0, s1)
• Add specific testing flags and check static result + static made jump
• check docs
• strict=True

Review the remaining unchecked items in the checklist, beginning with the path API change from evaluate(self, t0, t1) to call(self, s0, s1), the documentation check, and strict=True. Confirm the relevant project code and docs, then consider the work done when those listed items are resolved and the project checks pass.