It's not needed.

Grammar rules have 2 variants: ones that are attached to a specific
compile function which is called to compile that grammar node, and ones
that don't have a compile function and are instead just inspected to see
what form they take.

In the compiler there is a table of all grammar rules, with each entry
having a pointer to the associated compile function.  Those rules with no
compile function have a null pointer.  There are 120 such rules, so that's
120 words of essentially wasted code space.

By grouping together the compile vs no-compile rules we can put all the
no-compile rules at the end of the list of rules, and then we don't need
to store the null pointers.  We just have a truncated table and it's
guaranteed that when indexing this table we only index the first half,
the half with populated pointers.

This patch implements such a grouping by having a specific macro for the
compile vs no-compile grammar rules (DEF_RULE vs DEF_RULE_NC).  It saves
around 460 bytes of code on 32-bit archs.

In these cases the heap is anyway used to create a new object so no real
need to use the C stack for iterating.  It saves a few bytes of code size.

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.