[WIP] Add support for nested hooks.

I've marked this as WIP because for some reason `before_each` seems to be nesting fine, but `after_each` aren't nesting. The assertion in `tests/nested_hooks.rs` line 11 should fail but isn't being executed at all. I'm probably making some obvious mistake here.. any ideas?
Good idea! Will do after I've figured out the `after_each` issue.