toolbox.sh: Allow modules to implement their own interface

toolbox.sh: Allow modules to implement their own interface

The interface/implements mechanism does not allow a module to
implement methods of the interface that it declares. This makes it
impossible to write modules that extend other modules.

This commit changes the interface mechanism so that a module may
provide methods of the interface that it declares. This is done by
renaming module functions if they clash with names of the interface's
method stubs.