Use a cargo workspace for better modularity

Currently, there is only a `gateway` crate, but in the future,
likely more will be added to handle db stuff, abstractions, etc once
I actually start implementing the city portal logic itself.

src/schema.rs

@@ -1,8 +0,0 @@
-use crate::resolvers::QueryRoot;
-use async_graphql::{EmptyMutation, EmptySubscription, Schema};
-
-pub type AppSchema = Schema<QueryRoot, EmptyMutation, EmptySubscription>;
-
-pub fn build_schema() -> AppSchema {
-    Schema::build(QueryRoot, EmptyMutation, EmptySubscription).finish()
-}