Add an `__eq__` method that calls `equinox.tree_equal` under the hood

[LouisDesdoigts]

This would allow pytree1 == pytree2, which isn't necessarily important but could be useful as a way to abstract the need for equinox imports.

[LouisDesdoigts]

Looks like this already exists: https://github.com/patrick-kidger/equinox/blob/main/equinox/module.py#L258