+ StgPtr allocatePinned(nat n) Allocates a chunk of contiguous store
+ n words long, which is at a fixed
+ address (won't be moved by GC).
+ Returns a pointer to the first word.
+ Always succeeds.
+
+ NOTE: the GC can't in general handle
+ pinned objects, so allocatePinned()
+ can only be used for ByteArrays at the
+ moment.
+