Better Flow type defs for field resolvers

[leebyron]

When writing resolvers for GraphQL types in a flow-typed environment, the Flow types should aid as much as possible.

• Do not require casting through (: any) for typical behavior.
• Proper typing of context
• Proper typing of source
• Proper typing of return
• Proper typing of arguments

[leebyron]

This was originally inspired by discussion in #554.

[jamiehodge]

@leebyron is there any progress on this meta issue? It would really helpful to get end-to-end typing of the graph and the possibility of leveraging that in the client.

[jamiehodge]

@leebyron are you still working on this issue? Is there something I or others could help with?

[corydeppen]

Has there been any additional consideration for what it will take to move forward with the remainder of the typings for resolvers?