Add a simple serialization mechanism.

The `Serializer<T>` class implements a binary serialization that
can write to (`serialize`) and read from (`deserialize`) a byte
buffer.  Also added convenience routines for serializing
a list of arguments.

This will mainly be for testing, specifically to transfer data to
and from the GPU.

Eigen/Core

@@ -166,6 +166,7 @@ using std::ptrdiff_t;
 #include "src/Core/util/XprHelper.h"
 #include "src/Core/util/Memory.h"
 #include "src/Core/util/IntegralConstant.h"
+#include "src/Core/util/Serializer.h"
 #include "src/Core/util/SymbolicIndex.h"
 
 #include "src/Core/NumTraits.h"