add a bunch of bounds checking to pytree

Pull Request resolved: https://github.com/pytorch/executorch/pull/7654

It's possible to pass arbitrary string input to pytree from Python; let's not have a bunch of low-hanging memory safety issues.
ghstack-source-id: 265152272
@exported-using-ghexport

Differential Revision: [D68166303](https://our.internmc.facebook.com/intern/diff/D68166303/)

Co-authored-by: Github Executorch <github_executorch@arm.com>