Perf

As suggested on the [mailing list](https://groups.google.com/forum/#!topic/julia-users/bzhhJCGLxCM), make the globals const. (this seemed to speed up median solve by 30% and making it faster than python).

I tried to replace "total overkill" dict in solve, but couldn't get it working,,,

TODO update readme to reflect this (Have a feeling there will be some more perf to come, e.g. type stability.)