Harden the CAPI include files a bit

Remove some definitions that should no longer be in there
(structs for which we have Pyston-internal definitions).

Instead of doing "typedef void PyIntObject", do
"struct _PyIntObject; typedef struct _PyIntObject PyIntObject;"
which prevents spurious things like type conversions or
sizeof(void)'s.
Add structseq.{c,h} and sliceobject.h

Haven't added structseq.c to the build quite yet