func (s *Set) Has(m Multihash) bool {
	_, ok := s.set[string(m)]
	return ok
}
Has returns true if the element is in the set.
package multihash
type Set struct {
	set map[string]struct{}
}
func NewSet() *Set {
	return &Set{
		set: make(map[string]struct{}),
	}
}
func (s *Set) Add(m Multihash) {
	s.set[string(m)] = struct{}{}
}
func (s *Set) Len() int {
	return len(s.set)
}