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
}