* examples/download.scm: a loose `gnunet-download' clone.
* system/foreign/unions.scm: add the possibility to specify #f as a
union variant to get a padding of the
size of the union.
* gnu/gnunet/fs/progress-info.scm: just adapted to the modification to
unions.scm.
* gnu/gnunet/fs/uri.scm: add a few utility functions: `parse-uri' and
`uri-file-size'.
* gnu/gnunet/fs/fs.scm: add `start-download` and `stop-download`.