* guix/scripts/publish.scm (show-help, %options): Add '--compression'.
(<compression>): New record type.
(%no-compression, %default-gzip-compression): New variables.
(%default-options): Add 'compression' key.
(narinfo-string): Add #:compression parameter and honor it.
(render-narinfo): Likewise.
(render-nar): Likewise.
<top level>: Add call to 'declare-header!'.
(swallow-zlib-error): New macro.
(nar-response-port): New procedure.
(http-write): Add call to 'force-output'.  Use 'nar-response-port'
instead of 'response-port'.  Use 'swallow-zlib-error'.
(make-request-handler): Add #:compression parameter and honor it.  Add
"nar/gzip" URL handler.
(run-publish-server): Add #:compression parameter and honor it.
(guix-publish): Honor --compression.
* tests/publish.scm (http-get-port, wait-until-ready): New procedures.
<top level>: Run main server with "-C0".  Call 'wait-until-ready'.
("/nar/gzip/*", "/*.narinfo with compression"): New tests.
* doc/guix.texi (Invoking guix publish): Document it.