Refactor and add mathcheck

I wonder whether it would make sense, since now there are several source files, to create a ```src``` folder and a ```tools``` folder. 

src/
   * prop_opt_eq_unopt.lua
   * prop_pfluamath_eq_libpcap_math.lua
   * prop_trivial.lua
   * utils.lua

tools/
   * pflang_math.lua
   * pflua-quickcheck
   * pflua_ir.lua

In the ```tools/``` folder I'd remove the ```.lua``` extension and use dash instead of underscore (to be consistent with the ```tools/``` folder in pflua).