In JSON, marshal byte slices to strings using the escape syntax.

We currently encode byte slices to base64 encoded strings. We should instead use the escape syntax. Unfortunately, making this work _correctly_ may be kind of a pain.

Start by locating the JSON marshaling implementation for byte slices and any existing tests covering the current base64 output. Trace the required escape syntax and its edge cases before changing behavior; done means byte slices use that syntax instead of base64, with tests demonstrating correct encoding.

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