* 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.
* gash/config.scm.in:
* configure: Use it to generate gash/config.scm
* .gitignore: Ignore generated file.
* gash/gash.scm (display-version): Use it.
* build-aux/build-guile.sh: Compile it.