jsonfmt: Move arg parsing into XS

Going to need a way to pass arguments into the XS function anyway, so
might as well do the entire arg parsing step in XS while we're at it.
Provides a significant speedup for tiny inputs as well, but I don't find
that too interesting.

FU/Util.pm

@@ -6,12 +6,6 @@ use Exporter 'import';
 
 our @EXPORT_OK = qw/json_format/;
 
-
-sub json_format($val, %opt) {
-    my $r = FU::XS::json_format($val);
-    # XXX: Does this go over the bytes? If so, not setting SvUTF8_on() in the first place would be much faster.
-    utf8::encode($r) if $opt{utf8};
-    $r
-}
+*json_format = *FU::XS::json_format;
 
 1;