Add a gql!() generator macro.

[Yatekii]

Why are we making this change?

This PR introduces a new gql()! proc_macro, which allows the user to define adhoc queries, mutations and subscriptions like so:

cynic::gql! {
    mutation create_user($username: String!, $password: String!) {
        createUser(input: { username: $username, password: $password })
    }
}

This then generates a Rust function and all the necessary types & imports much like the cynic generator. Which then can automatically do the request for you.
This allows for very easy and smooth RPC call generation.

What effects does this change have?

• Adds more precise error messages to the parser parts of cynic (needs a custom graphql-parser fork for now).
• Adds a new gql!() macro to generate GraphQL RPC calls.

The code state is quite a mess yet I think and I am very glad about all input how we can make this a smooth sailing :)