Switch almost everything to use vregs when possible

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

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.

fix some scipy segfaults

rearrangeArguments added a extra ref to numpy array in when call numpy
resize function. fixes issue #1275

If rewriter encounter a situation which can't be handled. It will set
the `failed` to `true`. In this situation, just stop to emit code. And
also don't check some assert.

Optimize the llvm refcounter pass

Remove the "CPython executable parser"

Pyston call CPython executable to generate CPython AST. Remove this
"parser", use CPython Parser module by default. Pass "-x" to enable
libpypa parser.

- was accidentally inserting elements into the map when we didn't need to
- the maps are small so use a data structure optimized for that

ICs: variable size IC slots