The extra 4 slots correspond to the iterator object stored on the stack.

So that the "for x in it: ..." statement can now work without using the
heap (so long as the iterator argument fits in an iter_buf structure).

Allows to iterate over the following without allocating on the heap:
- tuple
- list
- string, bytes
- bytearray, array
- dict (not dict.keys, dict.values, dict.items)
- set, frozenset

Allows to call the following without heap memory:
- all, any, min, max, sum

TODO: still need to allocate stack memory in bytecode for iter_buf.

Per:

$ git log modlwip.c |grep ^Auth | sort | uniq -c
      9 Author: Damien George
      2 Author: Galen Hazelwood
     43 Author: Paul Sokolovsky

To run the testsuite on small ports.

As a "more basic" builtin iterator, present even in smaller ports.

This patch changes the threading implementation from simple round-robin
with busy waits on mutexs, to proper scheduling whereby threads that are
waiting on a mutex are only scheduled when the mutex becomes available.

This improves efficiency of GIL release within the VM, by only doing the
release after a fixed number of jump-opcodes have executed in the current
thread.

It's more efficient using the system mutexs instead of synthetic ones with
a busy-wait loop.  The system can do proper scheduling and blocking of the
threads waiting on the mutex.

Depending on the thread scheduler, a busy-wait loop can hog the CPU and
make the tests very slow.  So convert such loops to loops that have an
explicit sleep, allowing the worker threads to do their job.

For small ports which don't have all features enabled.

This was a debug output for initial porting, breaks tests.