Give top-level methods some rules

1. explicit `self` is forbidden in top-level method
2. top-level methods can only be called without a explicit receiver

In the code, there is workarounds for 3 hard-coded special cases:
* `sys` and `exit`: because, for an unknown reason, intern method cannot be top-level. #493
* `args` because it is currently both in Sys and Object thus has a crazy status. #461

Note: the first commits fix a bug in the `for` where the implicit `iterator` method was resolved with the `recv_is_self` flag always set to true.