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.
[FOLD] Cleanup of STAmount formatting functions. Remove boost::format
[WIP] Remove boost::format
[WIP] Remove boost::format
[FOLD] isSetBit should not be a macro
[FOLD] Remove boost::format
[FOLD] Reduce boost::format use