* gash/script.scm (and-terms, or-terms): New macros.
* gash/peg.scm (transform): Transform to use them.
(parse-): Parse && and || as binary term operator.
* test/20-and.sh: Test it.
* test/20-or.sh: Test it.
* check.sh (tests): Run it.
* gash/commands/sed.scm: New file.
* build-aux/build-guile.sh: Compile it.
* configure: Create script.
* gash/shell-utils.scm (with-atomic-file-replacement,
substitute*): New function, Import from Guix.
(substitute-port): New function.