Compiler for `GraphQL::Schema`

[amomchilov]

Motivation

Which creates a strongly-typed sig for #context, if the context class was customized via the context_class DSL.

For Shopify employees, please see the timeline for a link to a PR with a sample application of this compiler.

Implementation

I copied the GraphQLMutation compiler and edited it.

Tests

I copied the tests for the GraphQLMutation compiler and edited those, too. :)

[amomchilov]

You set the context class on the Schema, but you don't actually ever read it from the Schema 🤦

I'll rework this to add sigs to the correct classes, like GraphQL::Schema::Resolver, GraphQL::Schema::Mutation, etc.