Some of the build bots, still fail on this.  I suppose mine pass because
I enable C++11...

Anyway, remove these again.

This reverts commit ff92de76.

Revert that and remove ASSERT_POD. Let's see which bots are
unhappy with this configuration...

Towards possibly using Null pool for some nil objects.

Apparently our gcc-6.4.0 on Alpine Linux distribution doesn't like defining static const
inside a method, lets put that on outside the classes.

Trying to see if it fixes gcc-4.2 bots.

Don't know why I thought I should subtract one there...

llvm-gcc-4.2 bot had this problem:
hb-private.hh:812: error: initializer specified for non-virtual method 'void hb_vector_t<Type, StaticSize>::operator=(const hb_vector_t<Type, StaticSize>&) [with Type = hb_user_data_array_t::hb_user_data_item_t, unsigned int StaticSize = 1u]'

Removing the delete didn't work with a constructor. So, remove constructor.
Just disallow assignment.  Still better than nothing.

This reverts commit face7cf5.

Fully untested.