Be compatible with Cow ≥ 2.0.0

[Chris00]

Fixes #146

Other eyeballs looking at the changes would be useful as it is easy to miss something in the transformations.

Note: If mirage/ocaml-cow#99 got merged, some more changes will be necessary (but maybe for another release).

[AltGr]

Oh, that's great, thanks for doing this!
According to Travis, it failed to build for some reason on 4.01 -- probably because it got cow 2.0.1 instead of 2.2.0 ?

+ ocamlfind ocamlc -c -bin-annot -g -annot -package cmdliner -package opamfu.cli -package opam-lib.client -package cow -package omd -package str -I apalog -o o2wPackage.cmo o2wPackage.ml
File "o2wPackage.ml", line 329, characters 13-24:
Error: Unbound value Html.strong
Did you mean string?

[Chris00]

I have added a constraint on Cow ≥ 2.2.0

[samoht]

LGTM, thanks!