CPython: freelist txn should have 0 refcount

Same reason as previous commit, the tracing allocator complains when we
call PyObject_Del() during env_clear(). Now the ref count is maintained
the same way tupleobject manages its freelist.
CPython: make Env.close() equivalent to env_clear().

The functions were essentially identical already, except env_close()
wasn't clearing out the freelist.