Add simple stack implementation

plugins/commands/serve/mappers/internal/stack.rb

@@ -0,0 +1,45 @@
+module VagrantPlugins
+  module CommandServe
+    class Mappers
+      module Internal
+        # Simple stack implementation
+        class Stack
+          def initialize
+            @data = []
+            @m = Mutex.new
+          end
+
+          def include?(v)
+            @m.synchronize do
+              @data.include?(v)
+            end
+          end
+
+          def pop
+            @m.synchronize do
+              @data.pop
+            end
+          end
+
+          def push(v)
+            @m.synchronize do
+              @data.push(v)
+            end
+          end
+
+          def size
+            @m.synchronize do
+              @data.size
+            end
+          end
+
+          def values
+            @m.synchronize do
+              @data.dup
+            end
+          end
+        end
+      end
+    end
+  end
+end