In order to make the packages in this repo "reinstallable", we need to
associate source code with a specific packages. Having a top level
`/includes` dir that mixes concerns (which packages' includes?) gets in
the way of this.

To start, I have moved everything to `rts/`, which is mostly correct.
There are a few things however that really don't belong in the rts (like
the generated constants haskell type, `CodeGen.Platform.h`). Those
needed to be manually adjusted.

Things of note:

 - No symlinking for sake of windows, so we hard-link at configure time.

 - `CodeGen.Platform.h` no longer as `.hs` extension (in addition to
   being moved to `compiler/`) so as not to confuse anyone, since it is
   next to Haskell files.

 - Blanket `-Iincludes` is gone in both build systems, include paths now
   more strictly respect per-package dependencies.

 - `deriveConstants` has been taught to not require a `--target-os` flag
   when generating the platform-agnostic Haskell type. Make takes
   advantage of this, but Hadrian has yet to.

is used outside of the rts so we do this rather than just fish it out of
the repo in ad-hoc way, in order to make packages in this repo more
self-contained.

These are unexploded minds as far as the linter is concerned. I don't
want to hit in my MRs by mistake!

I did this with `sed`, and then rolled back some changes in the docs,
config.guess, and the linter itself.

Added a section about commandline arguments

Make validator.w3.org happy

Ooops, forgot about a dozen references to configure.in... :-}

- Update the old coding style document
- Move it to the commentary under the "RTS & libs" section