If I'm programming in the OCaml toploop and I want to use a package from the [OCaml standard library](http://caml.inria.fr/pub/docs/manual-ocaml/libref/) or [some other library](http://www.janestreet.com/ocaml/doc/core/index.html), how I can find out which .cma file to load? In the standard library for example, `String` is in `str.cma` and `Big_int` is in `nums.cma`, so the filenames are not discernible from the module name or description.

Is there an easy way to look up the correct file for a module?