support multiple *app* instances in a single process

[tychoish]

Would you be interested in considering a pull request that moved things around so that it'd be possible to host/coordinate multiple applications within a single process, looking through the code, I think that the only implementations that would need to change are:

• set-route
• start
• with-route

I think it'd be easy enough to either: create -with-app variants of these, that the existing functions could wrap or create a separate package with the alternate forms, and then re/export them through the existing package, potentially wrapping the parameterized app variants.

In either case, the current API and functionality would be preserved, and downstream users would not be impacted.

Is this something you'd be interested in considering!