add a utility method for sorting CID slices

fixes https://github.com/ipfs/go-cid/issues/46

slice.go

+package cidutil
+
+import (
+	"github.com/ipfs/go-cid"
+	"sort"
+)
+
+// Slice is a convenience type for sorting CIDs
+type Slice []cid.Cid
+
+func (s Slice) Len() int {
+	return len(s)
+}
+
+func (s Slice) Less(i, j int) bool {
+	return s[i].KeyString() < s[j].KeyString()
+}
+
+func (s Slice) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+func (s Slice) Sort() {
+	sort.Sort(s)
+}
+
+// Sort sorts a slice of CIDs
+func Sort(s []cid.Cid) {
+	Slice(s).Sort()
+}