let MAKE_SCHEME_CALLBACK... handle overloading
This change lets me use both of these functions in Break_alignable_interface: SCM find_parent (SCM); Grob* find_parent (Grob*); An alternative is to mangle one of the names manually, e.g. internal_find_parent.
clean up names, division between Scheme/C++
Thanks for the feedback on naming. Are the new callback names better? https://codereview.appspot.com/547340043/