Callback_arity states the type of callbacks stored in a bus. Using Callback_arity
is an implementation technique that allows callbacks to be defined as ordinary n-ary
curried functions, instead of forcing n-ary-variadic callbacks to use tuples. This
also avoids extra allocation.
When reading the bus interface, keep in mind that each 'callback is limited, through
create, to the types exposed by the variants in Callback_arity.