ie trying to start running real C modules.  The goal has always been to
run them without modification and without a proxy layer, and this is the first step.

The first issue is conversion between PyObject and our internal representation,
"Box" (needs to be renamed).  Once they have the same layout, some hacks were
needed to make them seem like the same thing, even though we can attach C++
methods on the Box side, but use PyObject in C code.

The next major issue was PyTypeObject, since the structure is directly exposed to
extensions.  I just added the similar fields to BoxedClass, so they can be
used interchangeably; the "real" ones are the BoxedClass ones, but we'll migrate
them incrementally.

There's also the issue that PyTypeObject's are typically created statically,
so I added registerStaticRootMemory to the GC interface.

Also add in a smattering of other API functions, and the _sha module works
(who cares if it's normally disabled).

The C API is quite constraining about choices of data structure implementation; it's
in direct conflict with the std::string API, for example.  For now, close our eyes
and let the C API modify the internal bytes of std::string.