Implementation of the `fsm` combinator

[kjh618]

(From the discussions on the Oct. 14 class) Instead of the fsm combinator itself being a compiler magic, would it be better to make it a thin wrapper around a magic Fsm, and comb? Then, fsm would be implemented like the following:

fn fsm(self, init, f) {
    let m_fsm = Fsm(init, f);
    self.comb(m_fsm)
}

[minseongg]

Good idea! I believe it is more ergonomic way to implement the fsm combinator.

Since it requires some compiler changes, we will do it later. Thank you for suggestion!