Execution::Next: add as_default: true option, run entire test suite with it

[rmosolgo]

use GraphQL::Execution::Next, as_default: true will make Schema.execute(...) use Execution::Next.

Expanding CI coverage to all files ... there could be a lot of loose ends to follow up here 😅

TODO:

• ResolveTypeStep: properly batch these. Initialize a step when needed here.
• FieldResolveStep shortcuts when there's an error in arguments. Right now it goes three ways: a conditional in build_arguments, an early conditional in execute_field, and outside calls to loaded_argument_unauthorized. The fact that this is here three times is totally a smell. How can this data structure and flow be improved?