* gash/guix-utils.scm: New file, partial import from Guix.
Throughout: Add `compress.'
* build-aux/build-guile.sh: Compile it.
* gash/config.scm.in: Support it by defining compressors.
* configure: Substitute them.
* gash/bournish-commands.scm (tar-command): Use it to have tar support
compression and decompression.