Improve calculation of JSON_INT_MAX

From https://github.com/udp/json-parser/blob/e6426ae/json.c#L49-L55. That value can be calculated mathematically using `2**((n * 8) - 1) - 1`. Which can also be done using the following bit shift:
```c
static const json_int_t JSON_INT_MAX = (1L << ((8 * sizeof(json_int_t) - 1))) - 1
```