Add `path` argument to other functions

Might make sense in these functions
* `json_wrap_scalars()`
* `json_keys()`
* `json_flatten()`
* `json_array_length()`
* `json_array_types()`
* `json_unnest_wider()`
* `json_unnest_longer()`

When there is `path` one needs
* `default` when element at path does not exist
* `wrap_scalars`?