inheritable d and getting rid of virtual_hook

[Oswald Buddenhagen]

moin,

dfaure and me were pondering the following possibility:

when d is protected (not private), it can be used for adding protected
virtual methods, too - exactly the purpose of virtual_hook. and it is
way nicer and less wasteful than the latter.

of course that means that we have to introduce protected d everywhere;
this includes extracting the Private class of base classes to *_p.h.
but we want that anyway, as it saves allocations.

anything wrong so far?
any takers?

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