Type inference for struct field via function bound

[kngwyu]

Enables

impl<T: Clone> S<T> {
    fn f(self) {
        let b = a.mem.clo~
    } 
}

In addition, this PR removes FnArgTypeVisitor.
Currently we parse function twice for getting ident and getting type, which makes code too complicated.
So in this PR, I changed to parse function only once.