gnu: racket-minimal: Bootstrap from C.

gnu: racket-minimal: Bootstrap from C.

This commit bootstraps the Racket compiler and runtime system from source,
including Racket CS as well as both variants of Racket BC. (One remaining
limitation is discussed in comments added to gnu/packages/racket.scm.)

In the process, it moves to building minimal Racket from the Git repository,
rather than the packaged source tarballs. The Git repository is slightly
better as the ``corresponding source'':

 1. A few packages especially closely tied to the Racket core implementation
    (like "compiler-lib", "base", and "racket-doc") are developed in the
    same Git repository. Having them use the same Guix origin, too, will
    help to keep them in sync.

 2. The top-level Makefile in the Git repository is an important
    ``script[] used to control compilation and installation.''
    In particular, it cooperates with the "distro-build" package to
    create the source tarballs and installers for a Racket distribution.
    (Racket supports a notion of custom distributions.)

 3. It is ``the preferred form ... for making modifications'' to the core
    Racket implementation.

Racket releases are tagged in the Git repository (e.g. "v8.1"). At the
beginning of each release cycle, a branch is created to stabilizer a version
for extra testing. Active development happens on the "master" branch.

* gnu/packages/racket-minimal-sh-via-rktio.patch: Adjust for extra directory
layer.
* gnu/local/racket.scm (cfg-flag:sh-for-rktio, cfg-flag:enable-lt,
cfg-flag:enable-racket, unpack-nanopass+stex,
%main-repo-main-distribution-pkgs): New private variables.
* gnu/local/racket.scm (racket-minimal)[source]: Use Git.
[source](snippet): Unbundle nanopass, stex, and libffi.
[native-inputs]: Use racket-bootstrap-chez-bootfiles, plus its
dependencies (for Chez, plus a Racket for bootstrap pig).
[arguments]: Revise extensively.
* gnu/local/racket.scm (racket-minimal-bc-3m, racket-minimal-bc-cgc): New
packages, hidden at least for now.
(racket-bootstrap-chez-bootfiles): Another new package, but this one is
especially likely to stay hidden.
* gnu/local/racket.scm (racket)[origin](snippet): Unbundle packages
developed in the main Git repository, but leave their links.rktd and
pkgs.rktd entries in place.
[native-inputs]: Add the main Racket Git repository.
[arguments](#:phases): Adjust 'unpack-packages to also unpack package
sources from the main Racket Git repository.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>