Schema::Subset: Don't build schema caches, sort directives in introspection

[rmosolgo]

To support lazy-loading types, this turns off eager-loading when Schema::Subset is enabled.

Part of #5014 and #4919

TODO:

• Document: Schema.from_definition used to retain unreferenced types, but it doesn't anymore.
• Run benchmarks vs master to make sure this doesn't introduce problematic latency. (I don't expect so -- the calls to Schema. methods are only called once per type per query.)
  • Memory usage was identical, runtime was within margin of error