Move json_pointer::parse_array_index() and its status enum to namespace detail
(json_pointer keeps a forwarding member), and let the json_view pointer
resolution use it instead of its own copy of the RFC 6901 index grammar. Both
accepted the same tokens with the same outcome. Also update the comment on
detail::decode(), which listed four of the five UTF-8 validators and now points
to the test that keeps them in sync.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>