GType: add accessor for instance private offset

GType: add accessor for instance private offset

Since instance private data is now always at a constant offset to the
instance pointer, we can add an accessor for it that doesn't also
require an instance.

The idea is that classes can call this from their class_init and store
it in a file-scoped static variable and use that to find their private
data on instances very quickly, without a priv pointer.

https://bugzilla.gnome.org/show_bug.cgi?id=698056