toolbox.sh: Implement naive interfaces and inheritance

toolbox.sh: Implement naive interfaces and inheritance

The ipc and uipc modules provide the same API to the module user,
but the user cannot easily switch between the two implementations
because toolbox modules do not have a notion of interfaces.

This commit adds a very primitive means for modules to declare and
implement interfaces: when a module declares an interface, a vtable
is allocated in the form of an associative array, and call stubs
are generated which call the functions referenced in the vtable.
A module that implements an interface can then override entries in
the interface's vtable.