Convert (to/from)JSONValue to opCast/constructor. See #1904.
Convert (to/from)JSONValue to opCast/constructor. See #1904.

This allows other similar conversions to be implemented later, in particular for `stdx.data.json`. It also enables `json.to!JSONValue` and `jsonvalue.to!JSON` conversions.

chalucha/vibe.d,rejectedsoftware/vibe.d,etcimon/vibe.d,Geod24/vibe.d,gedaiu/vibe.d,etcimon/vibe.d