Handoff structure for lilos

This implements a synchronous rendezvous structure, which lets a task pass a value to another task without extra copies or reserving storage space.

This used to be part of the core lilos API, but was extracted during the process of finalizing the lilos 1.0 version. It is currently separate from lilos because its API is not cancel-safe.

Despite not being cancel-safe, it's still quite useful. See the module docs for more details.