* guix/utils.scm (<progress-reporter>): New record type.
(call-with-progress-reporter): New procedure.
* guix/build/download.scm (dump-port*, rate-limited, progress-reporter/file):
New procedures.
(ftp-fetch, http-fetch): Use 'dump-port*'.
(progress-proc): Remove procedure.
* guix/scripts/substitute.scm (progress-report-port): Rewrite in terms of
<progress-reporter>.
(process-substitution): Adjust accordingly.
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