Support older versions of OCaml

[samoht]

Lost of distros still ship with 4.01, so it would be nice it the projects compile on these platforms.

(or maybe, it 's just a matter of removing https://github.com/OCamlPro/opam-user-setup/blob/master/opam#L33)

[AltGr]

is there an external preprocessor that handles {bla| |bla} strings ? Or even a script that could be used to generate a dirty compat patch ?

There are big snippets of code using it over the place, and those would probably need to be put in external files with some kind of adhoc expansion without the new string syntax. I don't think it would improve the simplicity/readability of the project...

[samoht]

Maybe use ocaml-crunch? (an example here or there)