Fix SIOF for error-bounds and put stdx into predicates namespace

[artem-ogre]

• Static error bound constants depend on static epsilon constant which lead to static initialization order fiasco
• Insert stdx into predicates namespace to avoid the potential issue when namespace stdx is already defined in the client code

[artem-ogre]

Hi William, I have also moved namespace predicates { to avoid including std-headers into it. Would you be able to take a look at this PR?