This adds a dependency on the str library. It should be added to `META.in`.

Also, the syntax used by `Str` is tied to OCaml and is not as popular as PCRE. Instead we could use either pcre-ocaml or ocaml-re (https://github.com/ocaml/ocaml-re) which is a pure OCaml implementation that supports the PCRE syntax among others.

That said, it doesn't matter very much for this simple feature. Since atdgen already depends on str, let's use it here too.