Switch to external stringset implementation.

impl/memory/stringset.go

Index: impl/memory/stringset.go
diff --git a/impl/memory/stringset.go b/impl/memory/stringset.go
deleted file mode 100644
index d803cb1bd065eb85c26f5050653c073b6e2d3505..0000000000000000000000000000000000000000
--- a/impl/memory/stringset.go
+++ /dev/null
@@ -1,39 +0,0 @@
-// 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 ""
-}