Use Write::write in FileProducer::resize
Use Write::write in FileProducer::resize

This way we don't need any `unsafe` marked code here.

Use `vec![0; s]` to zero fill efficiently, and `Write` methods to do the
copying.

AndyShiue/nom,meh/nom,pwoolcoc/nom,Geal/nom,Keruspe/nom,sourrust/nom