tuple: expose JSON go_to_key and go_to_index functions

They are needed in JSON path update for a case, when a final part
of the path may not exist, and go_to_path returns an error.

That case is '=' and '!' on not existing fields. For example,
in {'=', '[1][2][3]', 20} field [3] can be not existing.

For these cases JSON update will have its own implementation of
go_to_path allowing optional last field.

Needed for #1261