[adamw]

Follow-up to #292, #293 and #294.

• Arrays and objects in example, default, enum and const values are now decoded to their JSON text (a String, e.g. {"a":1}), not to io.circe.Json. This is the same as what tapir passes for JSON examples, and apispec-model doesn't depend on circe. The encoder parses the text back, so round trips don't change.
• The encoder no longer has a special case for io.circe.Json values. It was only added in Represent null example values as null, drop null defaults and examples #294 and isn't released.
• ExampleSingleValue docs: the value is as it appears in the document. A String can also hold the value's encoded text (e.g. a codec's output), which is written as the JSON it holds.
• SchemaComparator: the Float/BigDecimal check is now in a function over plain values, which is also used for the elements of ExampleMultipleValue.