Derive macro for tagged enums (GraphQLUnion)

[jmpunkt]

Implements derive macros for tagged enums which are translated into GraphQLUnion types. The following features are provided

• The macro implements GraphQLAsyncType.
• If all variants of the tagged enum have different types, then the derive macro implements std::convert::From<T> converter for these types.

Fixes: #586

[LegNeato]

Looks great! Thank you so much for the PR, we always love new contributors writing high-quality code. 🥂 🎉

A couple of small things:

• Please add a test that actually queries the generated GraphQLUnion. You can see existing query tests in https://github.com/graphql-rust/juniper/blob/master/juniper/src/tests.
• Please add this to the book in the union section so folks know this exists

If you are up to it interfaces are very similar to unions in GraphQL and that would be a great next place to contribute...though they are a bit more complicated than unions.

[jmpunkt]

Thanks for the review. Everything you mentioned is fixed. During the testing i encountered some issues. Therefore, skip and documentation annotations are disabled for enumeration fields. Here are some thoughts about these decisions and the implementation. If something is unclear or wrong, let me know.

• Documentation of variations is not possible in GraphQL? (not 100% sure)
• Skipping fields works in general but throws panics during runtime. For example, if a query returns a skipped variant of the enum. The derive macro should provide additional safety (compared to the other union implementation). If the user requires this behavior he can fallback to the other union implementation.
• Currently the std::convert::Into ensures that each variant has a unique type. Due to the GraphQL specification this must be done? (not 100% sure)

[LegNeato]

It looks like at least in js unions can have descriptions for the entire thing: https://graphql.org/graphql-js/type/#graphqluniontype and it appears that the union types can as well. But, that is Apollo and not the official spec. So let's leave descriptions and skip out.

[jmpunkt]

This PR should be ready to merge. Applied your feedback and added some minor tweaks.

[LegNeato]

Awesome, thank you again!