Mutually Recursive Modules

In section 5, the report says that mutually recursive modules must be supported:

> Modules may be mutually recursive

but then doesn't say how to use it:

> separate compilation of mutually recursive modules may require that imported modules contain additional information so that they may be referenced before they are compiled

Saying that a feature must be supported but not explaining how to use it is bizarre and probably should be changed. Here's three different options on how this could be changed:

* A) Change the wording to say that implementations, may but are not required to support mutually recursive modules
* B) Codify GHC's `.hs-boot` files into the report
* C) Say that implementations must fully support mutually recursive modules and say that GHC's implementation is buggy

In [Hazy](https://github.com/superstar64/hazy), I currently fully support mutual recursive modules without any additional hassle and completely ignore `.hs-boot` files, so I would prefer to avoid B.

Start with section 5 of the report and review the wording about mutually recursive modules alongside the discussion of separate compilation and GHC's .hs-boot files. Determine which of options A, B, or C the report should adopt, then update the relevant report text so the support requirement and usage expectations are explicit.

Written by the indexing model from the issue text.