* guix/nar.scm (write-file): Add case for type `symlink'.
(restore-file): Likewise.
* tests/nar.scm (random-file-size, make-file-tree, delete-file-tree,
with-file-tree, file-tree-equal?, make-random-bytevector,
populate-file): New procedures.
(%test-dir): New variable.
("write-file + restore-file"): Use `%test-dir' and `file-tree-equal?'.
("write-file + restore-file with symlinks"): New test.
* guix/nar.scm (&nar-error, &nar-read-error): New condition types.
(dump): New procedure.
(write-contents)[dump]: Remove. Use the one above instead.
(read-contents, write-file, restore-file): New procedures.
(%archive-version-1): New variable.
