Fast GF(256) Cauchy MDS Block Erasure Codec in C++.

This is the rewrite in (as much as possible) clean C++ of cm256.

cm256cc is a simple library for erasure codes. From given data it
generates redundant data that can be used to recover the originals.

Currently only g++ is supported, other versions of MSVC than Visual
Studio 2013 may work. Optimizations for both SSE3 (x86_64) and Neon
(armv7) are available.
