[struct_pack] remove concept of trivial_class_t/non_trivial_class_t

[poor-circle]

Why

They are outdated after we add layout info to trivial serialized type.

What is changing

remove concept of trivial_class_t/non_trivial_class_t

Example

This code is legal now.

struct person {
  int age;
  std::string name;
};
auto buffer = struct_pack::serialize(24, "Student"s);
struct_pack::deserialize<person>(buffer);