perl: add $unpacker->utf8 mode, decoding strings as UTF-8.

perl/t/15_utf8.t

@@ -0,0 +1,27 @@
+#!perl -w
+use strict;
+use Test::More;
+use Data::MessagePack;
+use utf8;
+
+my $data = [42, undef, 'foo', "\x{99f1}\x{99dd}"];
+my $packed = Data::MessagePack->pack($data);
+
+my $u = Data::MessagePack::Unpacker->new()->utf8();
+ok $u->get_utf8();
+$u->execute($packed);
+my $d = $u->data();
+$u->reset();
+is_deeply $d, $data, 'decoded';
+
+is $u->utf8(0), $u, 'utf8(0)';
+ok !$u->get_utf8();
+$u->execute($packed);
+$d = $u->data();
+$u->reset();
+my $s = $data->[3];
+utf8::encode($s);
+is_deeply $d->[3], $s, 'not decoded';
+
+done_testing;
+