add support for from_json/to_json member function

[yt-sun]

add support for from_json/to_json member function. for a example:

using nlohmann::json;
class A {
private:
  int a;
public:
  void from_json(const json&){...}
  void to_json(json&) const {...}
}
void fun(){
  A a;
  json j;
  j=a;
  j.get_to(a)
}

[nlohmann]

Please check the results of the CI and update to the latest develop branch.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gregmarr]

@yt-sun Please check the results of the CI and update to the latest develop branch.