Add an API for 'named services'

[brson]

We currently have two global tasks that are started on demand and run until the process ends. The memory profiler that digitalknight is working on will add another. Each of these must deal with an ugly unsafe API, and they each create their own scheduler, threads, native stacks, etc.

It would be nice to have a safe API for this that allows services to be named (possibly like the TLS does) and that lets all services share a single scheduler. Services would still need an option to create their own scheduler though, as uv does need its own.

I imagine this would work by having just a single 'named service service' use the current unsafe API, and letting it in turn be responsible for managing the service lifecycle.

It would also be good to come up with a way to let services be stopped when not in use.

[brson]

This type of global task should not be required in the new scheduler, so may be out of scope for std.

[brson]

Un-nominating.

[graydon]

no compelling use cases at this point, closing WONTFIX