Import ocaml module Zed into wip/ocaml-zed.

Zed is an abstract engine for text edition. It can be used to write
text editors, edition widgets, readlines, ... You just have to
connect an engine to your inputs and rendering functions to get an
editor.

Zed provides:

* edition state management,
* multiple cursor support,
* key-binding helpers,
* general purpose unicode rope manipulation functions.
Add ocaml-zed.