Replace macros with proc_macros

[theduke]

Replace all macros used for defining GraphQL types with proc macro implementations.

• graphql_interface (🔬 Make interfaces great again! #682, Redesign interfaces (#1000) #1009)
• graphql_object (Add impl derive for graphql objects #181, Graphql object proc macro #333)
• graphql_scalar (Redesign #[graphql_scalar] macro #1014, Redesign GraphQLScalar macros #1017, Implement ScalarValue derive macro #1025)
• graphql_union (ee9a82a, Codegen reimplementation for GraphQL unions #666)
• graphql_value (not necessary, Macro for InputValue creation #996)

This should probably be a two-step process: first implement proc-macro alternatives for everything and have tests for functional equivalence.
Then remove the old macros with a new breaking version.

[samuela]

Forgive me if this is a noob question, but what's the difference between macros and proc macros?

[tyranron]

@samuela

Forgive me if this is a noob question, but what's the difference between macros and proc macros?

If it's still relevant, see the official reference:

• Macros By Example
• Procedural Macros

TL;DR

Declarative macros:

• declared via pattern-match syntax
• guaranteed to be hygienic
• restricted in expresiveness to certain patterns

Procedural macros:

• declared as a separate crate
• hygiene is up to an implementor
• accept and produce raw tokens, so can generate arbitrary kind of stuff