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.

To make skippable.

This allows using the test runner for other scenarios than just
testing uPy itself.
The principle of comparing either to CPython or else to a .exp
file is really handy but to be able to test custom modules not
built into micropython.exe one needs to be able to specify the
module search path a.k.a MICROPYPATH.

Should work for QEMU networking with soon-to-merged upstream patch.

MicroPython guarantees '/' to be a path separator, so extra constant taking
precious ROM space are not needed. MicroPython never had such constant, only
one vendor port had it (now unmaintained).

Just one sample is updated with  on()/off() for now, there should be
remaining sample(s) showing .value() use (but more can be converted later,
as long as 1 or so good samples of .value() remains).