[iter] Change item_size to get_item_size()

By moving access to hb_static_size(Type) into a function instead of
a class-const, we can refer to iter types of incomplete types, which
come handy when a method of hb_array_t wants to return iterator
of hb_array_t.  That kind of stuff.   Next commit needs this to
build on clang...
2 files changed