convert thread stack to a more efficient representation

The stack is now represented as alternating type tags and values instead
of object references.  This removes the need to store stack primitives
on the heap.

src/types.def

@@ -27,6 +27,7 @@
   (uint16_t flags)
   (uint16_t offset)
   (uint16_t parameterCount)
+  (uint16_t parameterFootprint)
   (object name)
   (object spec)
   (object class)