Vector3

In snekobject.h:

1. Forward declare the snek_object_t struct at the top of the file. It will need to be used in a circular dependency between snek_object_t -> snek_object_data_t -> snek_vector_t.
2. Create a new struct called snek_vector_t that has three fields. Name the fields x, y, and z. Each field should be a Sneklang Object pointer (snek_object_t *).
3. Add a new enum value to the snek_object_kind_t enum called VECTOR3.
4. Add a new snek_vector_t field (the struct type you just created) to snek_object_data_t called v_vector3.
5. Declare the new_snek_vector3 function.

In snekobject.c, complete the new_snek_vector3 function:

1. If any of the inputs are NULL, return NULL.
2. Allocate memory for a new pointer to a snek_object_t, and if the allocation fails return NULL.
3. Set the kind field to the appropriate enum.
4. Initialize the v_vector3 field of the new snek object so that its x, y, and z members point to the input objects (for example, by creating a snek_vector_t with those fields and assigning it to v_vector3).
5. Return the pointer to the new snek object.