Kernel SDK: rename store_list_get to store_list_get_nth

store_list_get host function does not exist - rather store_get_nth_key does instead.

NB the return of store_get_nth_key should return the subkey name, without being prefixed by /. This means that the return is not a valid path, and therefore we need to either:

• prefix it with / ourselves, to make it a valid path before returning
• add a step type to path.rs, can be appended onto a path, and when the appending happens we add the / between the old path, and the step.

NB the return of store_get_nth_key can be empty - which means that the index corresponds to a value - this should be returned as an Error/None variant.