[struct_pack][feat] add support for user_defined type id

[poor-circle]

Why

Allow user add type id to distinct different c++ type with same struct_pack type by different hash.

What is changing

Now user can defined a public static member struct_pack_id, this ID will be recorded in struct_pack type string to change hash code. This help user distinct different c++ type with same struct_pack type. User can also use this to avoid hash collision.

Example

struct foo_with_ID {
  std::vector<int> a;
  std::list<foo_with_ID> b;
  constexpr static std::size_t struct_pack_id = 0;
};

struct foo {
  std::vector<int> a;
  std::list<foo> b;
};

static_assert(struct_pack::get_type_literal<foo_with_ID>()!=struct_pack::get_type_literal<foo>());