Move Callback_wrapper class to separate file and simplify

Move Callback_wrapper class to separate file and simplify

Callback_wrapper::make_smob now is only templated on the address of
the (static) trampoline function.  Moving the trampolining to the
actual functions in question makes callbacks quite more versatile and
transparent and obviates the previous need for friend declarations due
to mixing internals of other classes into the Callback_wrapper
definition.

[Carl, 2016-04-28 14:22:00]

LGTM.  Looks like it is clarifying and simplifying things, and might even help
us with Guile2 migration.

[dak, 2016-04-28 14:36:07]

On 2016/04/28 14:22:00, Carl wrote:
> LGTM.  Looks like it is clarifying and simplifying things, and might even help
> us with Guile2 migration.

Actually, it's mostly preparatory work to pull the Acknowledger structures out
from C++ (and pointer-to-member stuff that has no equal representation in Scheme
engravers) and into Scheme.  Which would allow unifying the data flow of C++ and
Scheme engravers (currently, Scheme engravers acknowledge everything and then
sort it out manually).

I don't see it helping with Guile2.  The main advantage for me is that
Callback_wrapper now only does one thing without meddling in the internals of
other classes.