Import OCaml library Easy-format into wip/ocaml-easy-format.

This module offers a high-level and functional interface to the Format
module of the OCaml standard library. It is a pretty-printing
facility, i.e. it takes as input some code represented as a tree and
formats this code into the most visually satisfying result, breaking
and indenting lines of code where appropriate.
Add ocaml-easy-format.