building a bridge for 1.1

This provides a `to_1_1.ml` script (the compat function could be moved anywhere though) that was successfully tested to convert a 1.2 repo to 1.1-compatible format.

For this I used the 1.2-test branch of the ocaml/opam-repository repo, whis is derived from the master repo but with 1.2 additions and fixes added using other scripts in `/admin-scripts`