filters.go
func Map(fn func(string) string) Filter {
	return FilterFunc(func(arg Arg) error {
		for s := range arg.In {
			arg.Out <- fn(s)
		}
		return nil
	})
}
func Map(fn func(string) string) Filter {
	return FilterFunc(func(arg Arg) error {
		for s := range arg.In {
			arg.Out <- fn(s)
		}
		return nil
	})
}
// Map calls fn(x) for every item x and yields the outputs of the fn calls.