feat(util/hexfloat): add hexfloat to print exact/hexdecimal rep. of double number

 - std::hexfloat should do the job. However current g++ does not support
   it yet[1].
 - Instead, we use "%a" format specifier[2] + snprintf.

[1]: http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html
[2]: http://www.cplusplus.com/reference/cstdio/printf/