[RFC] Specify a directive syntax for encoding how to execute JavaScript resolvers

[captbaritone]

Talked with a potential Grats user who was asking if Grats could generate graphql-tools-style resolver maps instead of a GraphQL-js style GraphQLSchema. It's quite possible, but it got me thinking about a more explicit decoupling of the analysis that Grats does (what is your schema, and how do I execute it?) and tools that would make use of that analysis. I think there are some interesting possibilities if we can decouple those two things:

1. Relay Resolves could consume schema defined with Grats
2. Non-Grats tools could use Grat's codegen to build a GraphQLSchema
3. Someone (this new user) could write their own tool to produce resolver maps from the analysis Grats does
4. etc

This PR is a draft of that proposal for early feedback.

I'd be curious to hear thought on if this would be a valuable standard for people to be able to build to. If so, are there libraries/tools people have in mind that could be built? Any feedback on this initial draft?

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Replaces #151. Structured as a PR to facilitate inline feedback.

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