Switch to external stringset implementation.

impl/memory/stringset.go

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-package memory
-
-type stringSet map[string]struct{}
-
-func (s stringSet) has(value string) bool {
-        _, ret := s[value]
-        return ret
-}
-
-// add adds to the set and returns true iff the value was not there before (i.e.
-// it returns true if add actually modifies the stringSet).
-func (s stringSet) add(value string) bool {
-        ret := !s.has(value)
-        s[value] = struct{}{}
-        return ret
-}
-
-func (s stringSet) rm(value string) {
-        delete(s, value)
-}
-
-func (s stringSet) dup() stringSet {
-        ret := make(stringSet, len(s))
-        for v := range s {
-                ret.add(v)
-        }
-        return ret
-}
-
-func (s stringSet) getOne() string {
-        for k := range s {
-                return k
-        }
-        return ""
-}