gnu: commencement: Simplify Gash and Gash-Utils.

gnu: commencement: Simplify Gash and Gash-Utils.

This change does three things.  First Gash-Utils is updated to
0.1.0.  Then, the bootstrap Gash and Gash-Utils packages are
arranged to be built without using 'guild'.  Finally, instead of
using a binary 'tar' via 'bootstrap-executable' to extract Gash and
Gash-Utils, a self-extracting Scheme implementation of 'tar' and
'gzip' is used instead.

These three changes need to happen in concert to avoid breaking the
bootstrap process.

* gnu/packages/shells.scm (gash-utils): Update to 0.1.0; remove
'autoconf' and 'automake' from 'native-inputs'; delete 'arguments';
update the home-page, synopsis, and description.
* gnu/packages/commencement.scm (bootar): New variable.
(gash-boot): Replace 'tar', and 'xz' with 'bootar'; remove 'bash'
and 'guile-source'; do not use 'guile-build-system' but rather
'gnu-build-system' with some modified phases; disable tests.
(gash-utils-boot): Likewise.
(%boot-gash-inputs): Add 'bootar'; replace '%bootstrap-guile+guild' with
'%bootstrap-guile'.
(mes-boot): In the configure phase, do not reference 'GUILE_LOAD_PATH'
as it is no longer set.