FU::Util: Add gzip_compress() wrapper for libdeflate/zlib-ng/zlib

And use it for automatic output compression in FU, as (potentially)
faster alternative to Compress::Raw::Zlib.

Was also planning to maybe add support for Zstd or Brotli, but given the
performance of libdeflate, I'm not sure that's really necessary. Brotli
does tend to do a better job at compressing HTML, though.

FU/Util.pm

@@ -13,6 +13,7 @@ our @EXPORT_OK = qw/
     utf8_decode uri_escape uri_unescape
     query_decode query_encode
     httpdate_format httpdate_parse
+    gzip_lib gzip_compress
     fdpass_send fdpass_recv
 /;
 
@@ -367,6 +368,48 @@ This will not happen if your local timezone is UTC.
 =back
 
 
+=head2 Gzip Compression
+
+Gzip compression can be done with a few different libraries. The canonical one
+is I<zlib>, which is old and not well optimized for modern systems. There's
+also I<zlib-ng>, a (much) more performant reimplementation that remains
+API-compatible with I<zlib>. And there's I<libdeflate>, which offers a
+different API that does not support streaming compression but is, in exchange,
+even faster than I<zlib-ng>.
+
+There are more implementations, of course, but this module only supports those
+three and (attempts to) pick the best one that's available on your system.
+
+=over
+
+=item gzip_lib()
+
+Returns an empty string if no supported gzip library was found on your system
+(unlikely but possible), otherwise returns the selected implementation: either
+C<"libdeflate">, C<"zlib-ng"> or C<"zlib">.
+
+This function does not try very hard to differentiate between I<zlib> and
+I<zlib-ng>, so it may report that I<zlib> is being used on systems where
+C<libz.so> is, in fact, I<zlib-ng>.
+
+=item gzip_compress($level, $data)
+
+Returns a byte string with the gzip-compressed version of C<$data> at the given
+gzip C<$level>, which is a number between 0 (no compression) and 12 (strongest
+compression). Only I<libdeflate> supports levels higher than 9, for
+I<zlib(-ng)> the level is capped at 9. 6 is typically used as a default.
+
+Throws an error if no suitable library was found.
+
+=back
+
+This module does not currently implement decompression. If you need that, or
+streaming, or other functionality not provided here, there's
+L<Compress::Raw::Zlib> and L<Compress::Zlib> in the core Perl distribution and
+L<Gzip::Deflate> on CPAN.
+
+
+
 =head2 File Descriptor Passing
 
 UNIX sockets (see L<IO::Socket::UNIX>) have the fancy property of letting you