Add support for boost::json::value_from, boost::json::value_to

* `result`: `{ "value": t }` or `{ "error": e }`;
* `error_code`: `{ "category": "system", "value": 5, "message": "Access is denied" }`;
* `error_condition`: same;
* `error_code` wrapping `std::error_code`: `"category": "std:system"`;
* for `value_to`, when the category name is unrecognized (not one of "system", "generic", "std:system", "std:generic", "std:iostream"?, "std:future"?): deserialize to `unknown_category`, same value;
* maybe use the unknown category instead of "interop", too, without modifying the value.