Tidy-up to-dos

[patrick-kidger]

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