arena's "private" list of objects, so there might
be two references to that list.

64-bit machines that require pointers to be aligned (e.g. IA64)

entirely happy with the two new VISIT macros in compile.c, but I
couldn't see a better approach.

Explicitly clear all elements from arena->a_objects and remove
assert() that refcount is 1.  It's possible for a program to get a
reference to the list via sys.getobjects() or via gc functions.

No good way to extract output yet.

Replace the toy arena implementation with a real one,
based on allocating 8K chunks of memory by default.

Don't double check for NULLs and don't initialize if not necessary.
No functional changes.

simplifies a lot of error handling code, and fixes many memory leaks.