parseTrailers

    fn parseTrailers(reader: *Reader, amt_read: usize) (BodyError || std.Io.Reader.Error)!usize {
        const in = reader.in;
        const rn = try in.peekArray(2);
        if (rn[0] == '\r' and rn[1] == '\n') {
            in.toss(2);
            reader.state = .ready;
            assert(reader.trailers.len == 0);
            return amt_read;
        }
        var hp: HeadParser = .{ .state = .seen_rn };
        var trailers_len: usize = 2;
        while (true) {
            if (in.buffer.len - trailers_len == 0) return error.HttpHeadersOversize;
            const remaining = in.buffered()[trailers_len..];
            if (remaining.len == 0) {
                try in.fillMore();
                continue;
            }
            trailers_len += hp.feed(remaining);
            if (hp.state == .finished) {
                reader.state = .ready;
                reader.trailers = in.buffered()[0..trailers_len];
                in.toss(trailers_len);
                return amt_read;
            }
        }
    }