Vectors

We have arrived at our first container type! What makes VECTOR3 not like the other types is that it holds other snek objects. That is, it "references" them. They are contained in its x, y, and z fields.

When we create a vector and place objects in those fields, we need to increment the refcount of each of those objects! Otherwise, we might accidentally free its contents and be left with a vector holding garbage memory.

Assignment

1. Update the new_snek_vector3 function. It should refcount_inc each of its 3 objects.
2. Update the refcount_free function to handle vectors. It should refcount_dec each of its 3 objects (which will automatically free them if they hit 0).