pyke/pyke.[ch]: Make type skeleton structures be read-only.

We couldn't do this before because `INITTYPE_META' would write the
direct superclass to the `tp_base' slot in the skeleton before calling
`inittype'.  To make this work, then, we adjust `inittype' to take the
direct superclass as an extra argument and plug it into the newly
created heap-type; and `INITTYPE_META' needs adjusting to pass this
argument rather than trying to write to the skeleton directly.

Of course, then we need to actually mark the type skeletons as `const'.