move stealth code to its own folder

In order to anticipate the addition of groups of code, all highly related
stealth code has been moved to its own folder (and given the slightly more
convenient names of SX instead of StealthX). Soon I will be adding support for
colored coins and perhaps various other kinds of code groups, and I don't want
them all living in the same folder. So groups of code that are all highly
related will be moved into their own folders henceforth.