* guix/serialization.scm (write-bytevector): New procedure.
(write-string): Rewrite in terms of 'write-bytevector'.
* guix/store.scm (write-arg): Add 'bytevector' case.
(add-data-to-store): New procedure, from former 'add-text-to-store'.
(add-text-to-store): Rewrite in terms of 'add-data-to-store'.
* tests/store.scm ("add-data-to-store"): New test.
