maint: Implement an InterfaceCollection class to manage interfaces

Having an instance variable on class Interface is insufficient for
Interface::Indirector. This also changes the semantics of "Interface.interface"
to handle registration and loading actions, and for "Interface.new" to only
instantiate an Interface. Thus, consumers of the API should typically use
"Interface.interface", unless they have reasons to not want an interface
automatically registered.

Paired-With: Pieter van de Bruggen