Add (guix nar) and (guix serialization).

Add (guix nar) and (guix serialization).

* guix/store.scm (write-int, read-int, write-long-long, read-long-long,
  write-padding, write-string, read-string, read-latin1-string,
  write-string-list, read-string-list, write-store-path,
  read-store-path, write-store-path-list, read-store-path-list): Move to
  serialization.scm.
  (write-contents, write-file): Move to nar.scm.
* guix/nar.scm, guix/serialization.scm: New files.
* Makefile.am (MODULES): Add them.