Move auth package under registry package

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/auth/token/stringset.go

@@ -0,0 +1,35 @@
+package token
+
+// StringSet is a useful type for looking up strings.
+type stringSet map[string]struct{}
+
+// NewStringSet creates a new StringSet with the given strings.
+func newStringSet(keys ...string) stringSet {
+	ss := make(stringSet, len(keys))
+	ss.add(keys...)
+	return ss
+}
+
+// Add inserts the given keys into this StringSet.
+func (ss stringSet) add(keys ...string) {
+	for _, key := range keys {
+		ss[key] = struct{}{}
+	}
+}
+
+// Contains returns whether the given key is in this StringSet.
+func (ss stringSet) contains(key string) bool {
+	_, ok := ss[key]
+	return ok
+}
+
+// Keys returns a slice of all keys in this StringSet.
+func (ss stringSet) keys() []string {
+	keys := make([]string, 0, len(ss))
+
+	for key := range ss {
+		keys = append(keys, key)
+	}
+
+	return keys
+}