Shift Builder<P> from toInstance() to build()

Shift Builder<P> from toInstance() to build()

In order to be able to use Builder<P> as a marker
on generated Builders, it needed to shift to build()

All 'internal' uses for Builder have been changed
to use build().  For things used outside of yangtools,
a depracted toInstance() is maintained on child
interfaces/classes.

Change-Id: I43662296e328133a5b5267be54d407e2ad4b95e8
Signed-off-by: Ed Warnicke <eaw@cisco.com>