Currently, we have some functions that only apply to object which is more suitable to move as a member function of Object.

1. WrapPrivateKey() in session_impl, change to IsTpmWrappable() and WrapWithTpm() in class.

2. We have some pattern like

    string n = key_object->GetAttributeString(CKA_MODULUS);
    rsa->n = ConvertToBIGNUM(n);

Perhaps, create a helper `AllocBIGNUMFromAttr(key_object, CKA_MODULUS);` for that