Codegen reimplementation for GraphQL unions (#666)

- reimplement #[derive(GraphQLUnion)] macro to support:
    - both structs and enums
    - generics in type definition
    - multiple #[graphql] attributes
    - external resolver functions
- remove From trait impls generation for enum variants

- reimplement #[graphql_union] macro to support:
    - traits
    - generics in trait definition
    - multiple attributes
    - external resolver functions
    - GraphQLType implemetation for a raw trait object
    - GraphQLTypeAsync implemetation (#549)

- add marker::GraphQLUnion trait

- rewrite "2.5 Unions" section in Book (Juniper user documentation)

- rewrite `codegen` and `codegen_fail` integration tests for GraphQL unions

Additionally:
- re-export `futures` crate in `juniper` for convenient reuse in the generated code without requiring library user to provide `futures` crate by himself (#663)
- use unit type () as default context for EmptyMutation and EmptySubscriptions
- relax Sized trait bound on some GraphQLType and GraphQLTypeAsync definitions, implementations and usages

Author: tyranron

docs/book/content/types/unions.md

@@ -9,6 +9,8 @@ build = "build.rs"
 juniper = { path = "../../../juniper" }
 juniper_iron = { path = "../../../juniper_iron" }
 juniper_subscriptions = { path = "../../../juniper_subscriptions" }
+
+derive_more = "0.99.7"
 futures = "0.3"
 tokio = { version = "0.2", features = ["rt-core", "blocking", "stream", "rt-util"] }
 iron = "0.5.0"

docs/book/tests/Cargo.toml

@@ -0,0 +1,6 @@
+use juniper::graphql_union;
+
+#[graphql_union]
+enum Character {}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/attr_wrong_item.rs

@@ -0,0 +1,7 @@
+error: #[graphql_union] attribute is applicable to trait definitions only
+ --> $DIR/attr_wrong_item.rs:3:1
+  |
+3 | #[graphql_union]
+  | ^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

integration_tests/codegen_fail/fail/union/attr_wrong_item.stderr

@@ -0,0 +1,6 @@
+use juniper::GraphQLUnion;
+
+#[derive(GraphQLUnion)]
+union Character { id: i32 }
+
+fn main() {}