now this highlights the unsettled decision still the more,
as can be seen by all that unnecessary copying. Basically we move the
Diff into the lambda-closure, from there into an anonymous instance,
from there into the embedded Buffer in MutationMessage, which again
just happens to sit in the closure storage when the action is invoked.
And all of this copying just to move the DiffMessage for consumption
into the TreeMutator...
thus by #1066 we should really get rid of the MutationMessage class altogether!
actually I do not know much regarding the actual situation when,
within the Builder run, we're able to detect a change and generate
a diff description. However, as a first step, I'll pick IterSrouce
as a base interface and use a "generation context", which is to be
passed by shared-ptr
