add a Schema.surface method which removes external definitions

[queerviolet]

this PR introduces the Schema.surface method. .surface(retain?) removes all external definitions except those you tell it to retain. it does not handle removing @inaccessible, but can be used prior to an inaccessible removal step.

(note that if the intention is to use this prior to removing @inaccessible, you should probably retain@inaccessible ;) )

[changeset-bot]

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Latest commit: 2941681

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