guix hash: Add '--recursive'.

* guix/scripts/hash.scm (show-help): Add --recursive.
  (%options): Likewise.
  (guix-hash)[file-hash]: New procedure.  Honor --recursive.
  Use it.
* guix/nar.scm (write-file): Add missing field to the &nar-error
  condition raised upon unsupported file type; change its message to be
  more descriptive.
* tests/guix-hash.sh: Add tests with -r.
* doc/guix.texi (Invoking guix hash): Document --recursive.