Pre-RFC Local Wakers

[Matthias247]

@tvallotton

Context would get two additional methods: local_waker() to get a LocalWaker, and set_local_waker(&mut self, waker: &LocalWaker) to set it.

I think one of the questions that a full RFC should answer is whether Context should be mutable and the local waker should be set after creating the Context or whether

1. There is a new way to build Contexts (a Builder?) which allows to set the newly added optional arguments
2. Contexts wrap other existing Contexts, and augment them with new properties - as e.g. Golang's Context::With_timeout does. This might be in addition to 1) or a later follow-up