publish: Add support for zstd compression.

* guix/scripts/publish.scm (compress-nar)[write-compressed-file]: New
procedure.
Use it for 'gzip' and 'lzip'.  Add 'zstd.
(nar-response-port, string->compression-type): Add case for 'zstd'.
* tests/publish.scm (zstd-supported?): New procedure.
("/nar/zstd/*"): New test.
* doc/guix.texi (Invoking guix publish): Document zstd compression.
(Base Services): Add cross-reference to the above node.