serialization: Add 'fold-archive'.

* guix/serialization.scm (read-contents): Remove.
(read-file-type, fold-archive): New procedures.
(restore-file): Rewrite in terms of 'fold-archive'.
* tests/nar.scm ("write-file-tree + fold-archive")
("write-file-tree + fold-archive, flat file"): New tests.