publish: '--compression' can be repeated.

This allows 'guix publish' to compress and advertise multiple
compression methods from which users can choose.

* guix/scripts/publish.scm (actual-compression): Rename to...
(actual-compressions): ... this.  Expect REQUESTED to be a list, and
always return a list.
(%default-options): Remove 'compression.
(store-item->recutils): New procedure.
(narinfo-string): Change #:compression to #:compressions (plural).
Adjust accordingly.
(render-narinfo, render-narinfo/cached): Likewise.
(bake-narinfo+nar): Change #:compression to #:compressions.
[compressed-nar-size]: New procedure.
Call 'compress-nar' for each item returned by 'actual-compressions'.
Create a narinfo for each compression.
(effective-compression): New procedure.
(make-request-handler): Change #:compression to #:compressions.
Use 'effective-compression' to determine the applicable compression.
(guix-publish): Adjust handling of '--compression'.
Print a message for each compression that is enabled.
* tests/publish.scm ("/*.narinfo"): Adjust to new narinfo field
ordering.
("/*.narinfo with properly encoded '+' sign"): Likewise.
("/*.narinfo with lzip + gzip"): New test.
("with cache, lzip + gzip"): New test.
* doc/guix.texi (Invoking guix publish): Document it.

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