gnu: r: Rename to r-minimal.

* gnu/packages/statistics.scm (r): Rename to...
(r-minimal): ...this new variable.
(r-with-recommended-packages): Rename to...
(r): ...this.
* guix/build-system/r.scm (default-r): Reference r-minimal.
* gnu/packages/emacs.scm (emacs-ess)[inputs],
gnu/packages/machine-learning.scm (shogun)[inputs],
gnu/packages/python.scm (python-rpy2)[inputs],
gnu/packages/bioinformatics.scm (ribotaper)[inputs],
(couger)[propagated-inputs],
(roary)[inputs],
(rsem)[inputs],
(rcas-web)[inputs]: Change "r" to "r-minimal".