Named service support

[astavonin]

We can come up with a public API to create global, singleton tasks, possibly with the following signatures:

   unsafe fn register_named_service<T>(n: str, f: fn~(port<T>))
   unsafe fn get_named_service<T>(n: str) -> chan<T>

This would, if the named service doesn't exist, create a new task and execute the specified function. You would use this in each test to create or retrieve a global service to manage your state. The big reservation I have about this is that we can not currently make this interface type safe - we can't even check the types at runtime.

Description was copied from mail-list.

[brson]

Partially implemented in core::priv. Would benefit from #553. Requires #61 or reflection to be typesafe.

[Aatch]

This doesn't look like it's valid any more. Re-open if you disagree.