Don't use Interned<T> for DeliveryLabel

[mxgrey]

DeliveryLabel is used inside Service for users to instruct when multiple calls to the same service should queue up or preempt each other.

We've been using Interned<T> from bevy to implement DeliveryLabel, but we recently realized that Interned<T> intentionally leaks memory in order for Interned<T> to implement the Copy trait.

This memory leakage is okay for Bevy since Interned<T> is used for system labels which are bounded in their variety and will use very few of them. Conversely, we could see arbitrarily many DeliveryLabel instances created over the run of an application. Each run of a workflow could introduce a new DeliveryLabel value, so an application that runs for years could eventually leak an enormous amount of memory.

We should introduce our own alternative to Interned<T> which uses Arc instead of leaking memory. This will mean Service<T> can no longer implement the Copy trait, but this is not a crucial property for our use case.