Reducing json_t size

[dkoreshkov]

I suggest redefining json_t someday so that it becomes 8 bytes long instead of 16 where size_t takes 8.
Isn't 32 bits enough for a reference counter?

typedef struct json_t {
    json_type type;             // most likely 4 bytes
    volatile unsigned refcount; // should be 4 too
} json_t;