Make json_error_t opaque

Make json_error_t opaque

All decoding functions now accept a json_error_t** parameter and set
it to point to a heap-allocated json_error_t structure if an error
occurs. The contents of json_error_t are no longer exposed directly, a
few functions to do it have been added instead. If an error occurs,
the user must free the json_error_t value.

This makes it possible to enhance the error reporting facilities in
the future without breaking ABI compatibility with older versions.

This is a backwards incompatible change.