Change the underlying type of JSON integer from long to json_int_t

Change the underlying type of JSON integer from long to json_int_t

json_int_t is typedef'd to long long if it's supported, or long
otherwise. There's also some supporting things, like the
JSON_INTEGER_FORMAT macro that expands to the printf() conversion
specifier that corresponds to json_int_t's actual type.

This is a backwards incompatible change.