Clean ht-ob headers

Is this what you meant with the "/* TODO Remove */" in hb-ot.h?

I cleaned some of the header files.
In my humble opinion, an hb_ot_shape_* function shouldn't be in hb_ot_layout.h,
and same with a shape function in hb-ot.h

Move those two to a hb-ot-shape.h.

Maybe you would like more functions from hb-ot-shape.cc defined there as well. As you like :)

Let me know.