Refactored SelectionSet::split_top_level_fields

[TylerBloom]

This PR refactors SelectionSet::split_top_level_fields away from using Box<dyn Iterator>s. While not a performance issue (the function is called from exactly one place), it was a very smelly implementation, which required every item to be to be allocated in its own allocation before being yielded by the iterator.

Since the iterator is recursive, I had to just implement Iterator on an internal type rather than use the opaque type (which can't be recursive at all) returned by the function.

Lastly, I saw a couple of other Box<dyn Iterator>s that that could be turned into RPITs by using the Either type from the either trait. either is already a transitive dependency, so it adds no extra dependencies to the apollo-federation or apollo-router.

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• Documentation² completed
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• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

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[router-perf]

CI performance tests

• connectors-const - Connectors stress test that runs with a constant number of users
• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload