GraphQL Schema Audit

Audit one GraphQL schema and separate what stops it building from what is legal and costs something. Reports a type implementing an interface without declaring its fields, a reference to an undefined type, a union member that is not an object type, an interface nothing implements, and an absent query root. Separately reports a nullable list item type, which forces every client to handle three outcomes, and a deprecation with no reason. Unreachable types are named as dead weight.