opam.ocaml.org: 57/99 problem code

i've done cone to 57. problem from 99 problem list.

https://github.com/emil3566/99-ocamls-problem/blob/master/src/50-59/57.Layout%20a%20binary%20tree%20%283%29.ml

It's commented and i allow creators of procjet to change it on their needs. I'm just putting it here becuase it might be helpfull, cuase there is no code for this task. It uses Core.Std so it should be said somewhere :) . If u find it usefull, feel free to use it, or copy.