Add missing tp_as_mapping

Support attrwrapper as globals argument to execfile

It keeps timing out

Or rather, have execfile() be more permissive in what it accepts.
It may still abort when we get to pickGlobalsAndLocals() and we
discover that we can't actually execute in that particular attrwrapper.

only lookup __[set/get/del]slice__ for slice AST nodes

Switch almost everything to use vregs when possible

before this change
C()[slice(1,2)] = "a"
would call into `__setslice__` because it got handled as
C()[1,2] = "a"

This commits adds cpythons assign_slice/apply_slice and add rewriter support for them.

The VRegMap doesn't contain explicit "is-set" bits, so I originally
just had iteration return all values.

But all the uses of it ended up needing to have a default value to represent
"not set in map" and then iterating would explicitly check if the value was
that and then would skip it.

Now that behavior is moved into VRegMap itself.

And update sys_test to support apport overriding excepthook

A tricky side-effect I didn't think about is that
block-local vregs can now look "live" at the end of
their block.  When using names, there are no other uses/defs,
but if we reassign the vreg there can be.

ie don't allow calling it for ones where multiple names
will reuse the same vreg

The benefit here is indirect, of letting us get rid of some
of the compatibility shims for moving between vregs and names.

They were previously represented as strings, like in cpython,
but we want to treat them like names.

Maybe not the best place to start

rather than implicitly via being missing in the hashtable.

Track down an elusive reference leak

The pattern seems to be that we invalidate an IC (deregestering the decrefinfo),
but there is a stack frame in that IC and furthermore that stack frame will
get unwound via an exception.

The fix I did here is to not deregister the decrefinfo if num_inside>0.

capi: add the direct access macros back

mp_ass_subscript could become NULL and we would have crashed

deinitFrame: don't modify CAPI exception state

If a CAPI exception was set and we called deinitFrame in some rare cases calling the finalizers cleared the CAPI exception.
This would cause us to return NULL even though no exception is set #1273 (for LLVM functions compiled in capi mode)
Unfortunately I could not come up with a testcase which triggers it but I verified that it fixes the problem scipy triggered.