Recursive packs

[lpw25]

This RFC has been extracted from #11. I suspect that this part is more contentious than the rest of #11 so it makes sense to split it out into its own RFC. From the summary:

OCaml's module system has great facilities for structuring your code
in the large. It supports encapsulation, parameterisation and even
recursion. Unfortunately not all of these facilities are available
across multiple files. The -pack feature allows you to group
multiple files together under an interface, but there isn't really
a way to allow recursion amongst a group of files.

This proposal aims to support recursion amongst groups of files by
extending recursive modules to work on groups of files via the -pack
mechanism. It is an extension of the Functor packs proposal.