CObj: add optional warning if method returning a <: CObject doesn't use an implicit wrapper

[jokade]

As a best practice, methods returning a wrapped C object should always take an implicit wrapper, i.e.

def getFoo()(implicit w: CObjectWrapper[Foo]): Foo = extern

instead of

def getFoo(): Foo = extern

To help enforce this policy, it should be possible to activate warnings/errors if it is broken.