Fake Buf (Buf8) class, should be quite memory efficient

buf8.pl

class Buf8 {
    has Mu $!buffer;
    method BUILD() {
        $!buffer := pir::new__PS('ByteBuffer');
        1;
    }
    method new(*@codes) {
        my $new := self.bless(*);
        $new.set_codes(@codes);
        $new;
    }
    method set_codes(*@codes) {
        pir::set__vPI($!buffer, nqp::unbox_i(@codes.elems));
        for @codes.keys -> $k {
            pir::set__1Qii($!buffer, nqp::unbox_i($k), nqp::unbox_i(@codes[$k]));
        }
        self;
    }

    method list() {
        my @l;
        for ^nqp::p6box_i(nqp::elems($!buffer)) -> $k {
            @l.push: nqp::p6box_i(pir::set__IQi($!buffer, nqp::unbox_i($k)));
        }
        @l;
    }
    method decode(Str:D $encoding = 'utf8') {
        nqp::p6box_s $!buffer.get_string(
            nqp::unbox_s pir::perl6_decontainerize__PP($encoding)
        );
    }
}

use MONKEY_TYPING;
augment class Str {
    method encode(Str:D $encoding = 'utf8') {
        my $buf := Buf8.new;
        pir::set__vPs(nqp::getattr($buf, Buf8, '$!buffer'),
            pir::trans_encoding__ssi(
                nqp::unbox_s(self),
                pir::find_encoding__is(nqp::unbox_s(pir::perl6_decontainerize__PP($encoding)))
            )
        );
        $buf;
    }
}

my $x = 'foo'.encode('utf8');
say $x.list;
say 'bar'.encode.decode;