API in ATS for Simple Dynamic String

I modified Hanwen’s original API and put it here:

https://github.com/githwxi/ATS-Postiats-contrib/tree/master/contrib/sdstring

It is a tiny library by Salvatore Sanfilippo for his implementation of
redis.

It is a pretty nice alternative to linear strings (strptr) in ATS. I have
also added
a few template-based iterators to make it a bit more attractive.