Improve parsing to avoid redundant copying and stack allocation

I was getting an error with an SVG image that has a large embedded image inside it (using an xlink:href attribute with a `data:image/png;base64` value)—the error was happening in SVGKParser when trying to strncpy the xlink:href attribute. I realized that there should be no need to strncpy the data; instead NSString's initWithBytes:length:encoding can be used directly. I changed the code to do so, which should make it more efficient and also avoids the error I was getting.