Implements 'def f(*, a)' and 'def f(*a, b)', but not default
keyword-only args, eg 'def f(*, a=1)'.

Partially addresses issue #524.

This is a small optimisation to zero out only the additional bytes that
the caller did not ask for.

Few other strings move to core, but make depend on "io" module.

Share code with .strip(). TODO: optimize .rstrip().

This is CPython-compliant (except that CPython doesn't support buffer
protocol for str).

This in particular fixes writing str vs bytes.

Also add a few STATIC's to some compile functions that should have them.

Addresses issue #521.

Also add some more debugging output to gc_dump_alloc_table().

Now that newly allocated heap is always zero'd, maybe we just make this
a policy for the uPy API to keep it simple (ie any new implementation of
memory allocation must zero all allocations).  This follows the D
language philosophy.

Before this patch, a previously used memory block which had pointers in
it may still retain those pointers if the new user of that block does
not actually use the entire block.  Eg, if I want 5 blocks worth of
heap, I actually get 8 (round up to nearest 4).  Then I never use the
last 3, so they keep their old values, which may be pointers pointing to
the heap, hence preventing GC.

In rare (or maybe not that rare) cases, this leads to long, unintentional
"linked lists" within the GC'd heap, filling it up completely.  It's
pretty rare, because you have to reuse exactly that memory which is part
of this "linked list", and reuse it in just the right way.

This should fix issue #522, and might have something to do with
issue #510.

Qemu

objexcept: Don't store args tuple within exception object.

vm: Add rudimentary bytecode execution tracing capability.

To help with debugging issue like #510 for example.

Suggested by @lurch.

To avoid pointer-to-field GC problem.

Some BSD socket functions don't return error numbers in errno namespace, but
rather in other error namespaces. CPython resolves this by using OSError
subclasses for them. We don't do that so far, so there's ambiguity here.

Well, Python3 also defines an attribute for that, but that's bloat.

Exact behavior of typecodes may be not yet enforced.