ly_scm_list_t robustness

1. Make the empty() condition more consistent with the iterator's validity check. The iterator stops if it is advanced to an element that isn't a list. Similarly, a list is considered empty if the head doesn't refer to a pair.

2. Rename ly_scm_list_t::erase() to erase_at() because of an important difference from std::list::erase(). Add lots of comments.