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

[pdimov]

• result<T, E>: { "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.

[pdimov]

The JSON-RPC specification uses { "result": t } for the value case, instead of "value"; it also uses { "code": 5 } for the error code, instead of "value".

[pdimov]

The category in the std::error_code case should probably also be just "system" or "generic", so as to match the eventual value_from for std::error_code provided by Boost.JSON.