Lang team champions

[nikomatsakis]

This PR defines the lang-team champion role and revises our goals. I'm not sure that we've got it nailed yet. For one thing, I am championing perhaps a bit more than I would like (though it's not just about raw numbers, as some of the things I am championing are already fairly advanced, e.g. RTN).

This is a limited version of the champion role that I proposed. It basically means: this person is the main point-of-contact for the lang team. They are empowered to make decisions. They are expected to post regular updates that will be reviewed by the broader team. If concerns arise, the champion is the one that decides what to do in response -- though people with concerns may escalate to a blocking concern if they disagree, but that should be done with caution.

Champions (summarizing):

Proposal or proposal group	Champion	Impl owner and notes
Macro rules proposals	@joshtriplett	wg-macros; I removed discussion of stabilization
C++ interop	@tmandry
Ergonomic RC	@nikomatsakis	I removed RFC decision, I think the best focus short term is experimentation, and I've been working with @spastorino on that
Async: implementable trait aliases	@tmandry to champion	@compiler-errors maybe driving implementation
Async: pin ergonomics	@tmandry to champion	@eholk driving implementation
Async: RTN	@nikomatsakis to champion	@compiler-errors has finished up impl, stabilization report needed
Const traits	@nikomatsakis to champion	const generics folks to drive, eg @oli-obk @fee1-dead @compiler-errors
Async: sync generators	@tmandry to champion	@eholk to drive
Unsafe fields	@scottmcm to champion	@jswrenn to drive
GPU offloading	@traviscross to champion	I don't think we have any real end-user expectation here
Externally implementable items	@nikomatsakis to champion	I expect @m-ou-se to operate fairly independently here

These champion assignments have not all been confirmed yet.

Rendered

[nikomatsakis]

(Worth pointing out that we can reassign these champions post RFC if we want to rejigger, the important thing is not to far outstrip our capacity.)

[traviscross]

I'll take pin ergonomics, if @tmandry wants. You two volunteered for quite of lot of these.

[nikomatsakis]

@traviscross I'm gonna merge but let's discuss you me and tmandry. Could be good, I agree we need to spread out more.