The [to_list] function comes from the Container interface. So if
you're wondering why you can't see it in core_string.mli, it's because
at the top of that file we say "include Container.S0". If you're
wondering why it's not in the OCamlDocs documentation, the reason is
similar: you should be able to click on the Container link near the
top of the page to see that signature. (That being said, OCamlDocs
doesn't cope too well with webs of includes and functors, so I
wouldn't be totally surprised if this didn't work completely well.)