Decrement and free

Now things are going to get a touch more complicated. In a refcounting GC, the interesting stuff happens when the refcount equals 0. That's when the garbage gets collected.

When the refcount reaches 0, there are no references to this object anymore! So, we need to free the memory automagically -- that's the whole point of a garbage collector.

For our first pass at this, we will only handle integers, floats and strings to keep things simple. As we go a little further, we'll implement it for the rest of the SnekObject types.

Assignment

1. Complete the refcount_dec function:
   1. Handle NULL objects gracefully.
   2. Decrement the refcount
   3. Then check if the refcount has reached 0, and if so, call refcount_free on the object.
2. Complete the refcount_free function:
   1. If the object is an INTEGER or a FLOAT, free the object itself (we don't need to worry about the data inside the object because it's stored directly in the object).
   2. If the object is a STRING, free the char* data inside the object, then free the object itself.
   3. If it's any other type, do nothing for now.