Moving a number of C++ constructs to SCM

[David Kastrup]

This is a sequence of patches.  The first one is just the same as the
already submitted issue 4840 (so that this can be tested on master):
it did not make sense to disentangle the bulk of this work from that
change.

If you point "git am" at the (sorted) sequence of saved mail messages,
it should apply the patches cleanly.

Patches bear the titles:

[PATCH 1/5] Issue 4840: Make Translator/Performer/Engraver abstract
[PATCH 2/5] Add Callback0_wrapper and Callback2_wrapper class
[PATCH 3/5] Add Method_instance class
[PATCH 4/5] Replace Engraver_dispatch_entry with Method_instance
[PATCH 5/5] Replace Translator_method et al

The main point is bringing the engraver structure close enough to
Scheme that Scheme engravers can work just as efficiently as C++
engravers.  At the current point of time, Scheme engravers acknowledge
_all_ grobs and then redistribute them to the Scheme engraver
acknowledgers manually instead of making use of the Dispatcher
classes.

These changes are a large step towards making Scheme acknowledgers
compatible with the framework connecting Dispatcher instances.