[1.10] Schema.build_from_definition should make classes

[rmosolgo]

This is a pretty big change for anyone using .from_definition to build their schemas. Now, .from_definition builds a class-based schema out of dynamically-created, anonymous classes.

That means, at least, that the ctx passed into resolvers will be a GraphQL::Query::Context instead of a GraphQL::Query::Context::FieldResolutionContext. Resolvers won't have access to metadata anymore. Maybe we'll need a followup to somehow support extras.