jsmntok_t with smaller size type

[jcecconi]

Sometimes, in embedded devices like the ESP32, using the 'int' type implies using 32 bits, which is often unnecessary. If you then need to use an array of this struct, it could consume a lot of stack memory. This is why I propose using uint16_t in the following way:

#Iifdef USE_JSMNTYPE_UINT16
typedef struct jsmntok {
jsmntype_t type;
uint16_t start;
uint16_t end;
uint16_t size;
#ifdef JSMN_PARENT_LINKS
uint16_t parent;
#endif
} jsmntok_t;
#endif