math/fxt: upgrade 2012.06.18 -> 2015.07.17
A substantial number of algorithms have been added, much in the
combinatorial section, but also quite a few others.
Very many routines have been improved (optimizations and
handling corner cases).
Everything (well, all demos) pass valgrind, so a handful of bugs have
been squished that are next to impossible to find otherwise.
Apart from that, a real and ongoing effort has been made for readability.