New Zero struct implements "compare with zero."

* A new empty struct allows you to make classes comparable with 0, so
  you can use constructors like x < Zero() or z != Zero().

  Please see beast/utility/Zero.h for more information.

* Tiny fix to beast/container/tests/hardened_hash.test.cpp
  which wasn't referring to all the modules it used.

* Add new BEAST_CONSTEXPR to handle Windows/C++11 differences
  regarding the constexpr specifier.