Add `path` argument to other functions

[mgirlich]

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?