Stdlib: add supports for encoding/json

[xhd2015]

Specifically, the json.newTypeEncoder. When it returns unsupported function, we can just silently ignore that field.

Here is an example when trying to marshal a stack trace with cancelFunc unmarshalable:

error:json: error calling MarshalJSON for type *sorted_map.SortedMap: json: error calling MarshalJSON for type *sorted_map.SortedMap: json: error calling MarshalJSON for type *sorted_map.SortedMap: json: error calling MarshalJSON for type *sorted_map.SortedMap: json: error calling MarshalJSON for type trap.object: field _r1: json: unsupported type: context.CancelFunc