build: Improve information density and appearance of download progress output.

* guix/build/download.scm (seconds->string): New function.
  (byte-count->string): New function.
  (progress-bar): New function.
  (throughput->string): Remove function.
  (progress-proc): Display base file name, elapsed time, and progress bar.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>