Support iterable return types

[blakeembrey]

GraphQL uses iterall and supports Iterable return values. This PR changes all return types to be iterable and keeps input types as arrays.

[jasonkuhrt]

Thanks @blakeembrey

• Could you give a motivating use-case?
• IIUC this proposes to allow any iterable to satisfactorially resolve a graphql schema list type, and graphql-js set this precedent
• I will play around a bit to better understand how loose this lets developers get with their resolvers.

[blakeembrey]

@jasonkuhrt I ran into this in my personal projects, where I tend to do more things in iterables lately with tools like https://github.com/blakeembrey/iterative. The major place this does get loose is strings, since everything else would work with graphql-js (https://github.com/graphql/graphql-js/blob/d55f6d364ada22c122c718ddf74951b20ef0b17b/src/execution/execute.js#L944 and https://github.com/leebyron/iterall/blob/76ce9a54b50fd9ae306e5f9fce6e2ab44618b65b/index.mjs#L164).