Implement C++ version of daff

[edwindj]

The original daff is written in the programming language Haxe, which can target many programming languages.

Currently the R package daff uses the javascript version of daff. Haxe however can also generate C++, so the lib may also be implemented using the C++ target. I don't know if/how the C++ version of daff should compile as a R package: to be found out.