Add custom uuid type. (c support only)
The uuid is a fixed length type (128-bit). Is is represented by 32 hexadecimal digits:

110e24f4-321b-11c4-0233-ff6c1a22000c

It can stored in c using a fixed 16-byte buffer and serialized.

uint8_t uuid[16] = {0x55,0x0e,0x84,0x00,0xe2,0x9b,0x41,0xd4,0xa7,0x16,0x44,0x66,0x55,0x44,0x00,0x00};

msgpack_pack_uuid(&pk, 16);
msgpack_pack_uuid_body(&pk, uuid, 16);

The serialized data is stored in the *type-data* format, using the reserved '0xd4' type.
+--------+--------+--------+--------+--------+--- ---+
|  0xd4  |               16 bytes              ...   |
+--------+--------+--------+--------+--------+--- ---+

The deserialized msgpack object type is 'MSGPACK_OBJECT_UUID'. The uuid can be copied using
the 'raw' pointer and size properties.

uint8_t uuid[16];

memcpy(uuid, obj.via.raw.ptr, obj.via.raw.size);