setTo method
Link a directed link with value, if from
and to
don't exist, they will be added
Optional tags
for setting tags
Implementation
void setTo(from, to, key, val, {List tags = const []}) {
final _f = _map_add_or_get(from, _newNode);
final _t = _map_add_or_get(to, _newNode);
_t.setFrom(_f);
_f.setToV(_t, key, val);
_f.setValTag(_t, key, tags);
}