Add support for rich effect functions via function pointers

[sonalmahajan15]

NilAway currently does not recognize function pointers as valid rich check effect functions, thereby reporting false positives.

type S struct {
    f func() (*int, error)
}

func testme(s *S) {
    v, err := s.f()
    if err != nil {
        return
    }
    _ = *v // FP: unassigned variable `v` dereferenced
}