Update to work with juniper master

[davidpdrsn]

It seems support for async GraphQL interfaces have landed in juniper master graphql-rust/juniper#682.

I think the time has come to start updating juniper-from-schema to support the latest master of juniper with all the changes thats been made. We should also migrate all generated code to use the juniper proc macros rather than the old and deprecated declarative macros.

I suspect this is going to be quite a big task and require some work. I urge users to remain patient. The work has begun!

Todo

• Docs section about specifically which versions of juniper are supported
• Make sure changelog is up to date
• Async resolver together with ownership: "as_ref"
• Support subscriptions Support subscriptions #129
• Support async Support async field resolvers #128
• Update changelog: Executor now requires two lifetimes but both can be elided 0bd0239
• Re-export juniper and use that to pin its version Re-export juniper from juniper_from_schema to make sure we're only using one version #125
• Cannot declare scalar with same name as built-in Give nice error when declaring scalar with same name as built-in #124

[davidpdrsn]

#122 is merged and we're now compatible with juniper master. At least all the tests are passing.

Still some work to be done around subscriptions and async. See the todo list in the top comment.

[davidpdrsn]

We now support async resolvers with a new @juniper(async: true) directive and the #[async_trait] attribute. You can find an example here.

[davidpdrsn]

The master branch now supports juniper 0.15. I've made a milestone for the things remaining before doing a proper release https://github.com/davidpdrsn/juniper-from-schema/milestone/1.