This commit moves the single anonymous and duplicated operation name
validation rules into the parser, allowing for a nicer format to be
exposed by it. It also adds better error messages to the parser.
The main change in this commit is changing the return type of
SubscriptionType::create_stream from
Future<Result<Stream<Result<Response>>>> to just Stream<Result<Json>>. I
also allowed the returned stream to borrow from self and context.
