Added VEX_STRINGIZE_SOURCE convenience macro

May be used to easily enquote the source code for a user-defined
function. It's advantage is that users will have benefits of syntax
highlighting and general readability of their code:

    VEX_FUNCTION(diff_cube, double(double, double),
        VEX_STRINGIZE_SOURCE(
            double d = prm1 - prm2;
            return d * d * d;
            )
        );

Got the idea from Boost.Compute (thanks, @kylelutz!)