It's supposed to mean "whether this type is heap allocated", but
it gets used as "is this a builtin/extension type".  We heap-allocate
our builtin types so we were setting HEAPTYPE, but this keeps on
causing issues.  I don't think anything actually uses HEAPTYPE to mean
it's on the stack or heap, so let's just set them all to be HEAPTYPE=0.

For better or worse, I ended up implementing this by changing the builtin
types from BoxedHeapClasses to BoxedClasses, which seems to make sense.
This had the effect of changing the existence of the tp_as_foo structs,
which get automatically created for HeapClasses but not for non-heap classes.
So most of the changes in this commit are trying to make sure that we
have those when CPython has them.