add Visitor pattern

visitor/README.md

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+## Visitor
+
+Visitor represents an operation to be performed on the elements of an object
+structure. It lets you define a new operation without changing the classes of 
+the elements on which it operates. The pattern has behavioral purpose and applies 
+to the objects.
+
+### When to use
+
+* an object structure contains many classes of objects with differing interfaces, 
+and you want to perform operations on these objects that depend on their concrete classes
+* many distinct and unrelated operations need to be performed on objects in an object structure, 
+and you want to avoid "polluting" their classes with these operations
+* the classes defining the object structure rarely change, but you often want
+to define new operations over the structure