New Zero struct implements "compare with zero."

* Zero lets classes efficiently compare with 0, so
  you can use constructors like x < zero or y != zero.
* New BEAST_CONSTEXPR to handle Windows/C++11 differences
  regarding the constexpr specifier.

beast/utility/Utility.cpp

@@ -31,3 +31,4 @@
 
 #include "tests/bassert.test.cpp"
 #include "tests/empty_base_optimization.test.cpp"
+#include "tests/Zero.test.cpp"