Added a freelist package.

internal/freelist/freelist.go

+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package freelist
+
+import "unsafe"
+
+// A freelist for arbitrary pointers. Not safe for concurrent access.
+type Freelist struct {
+	list []unsafe.Pointer
+}
+
+// Get an element from the freelist, returning nil if empty.
+func (fl *Freelist) Get() (p unsafe.Pointer) {
+	l := len(fl.list)
+	if l == 0 {
+		return
+	}
+
+	p = fl.list[l-1]
+	fl.list = fl.list[:l-1]
+
+	return
+}
+
+// Contribute an element back to the freelist.
+func (fl *Freelist) Put(p unsafe.Pointer) {
+	fl.list = append(fl.list, p)
+}