Re-using BytesStart byte vector

[jonhoo]

Currently, BytesStart will allocate a Vec<u8> the first time you push additional attributes for the element. That's unfortunate if you're emitting lots of BytesStart in a tight loop. It would be better if it was possible to re-use a BytesStart (specifically its internal Cow::Owned) across iterations. I think all that'd be needed would be two methods: set_name and clear_attributes:

impl BytesStart {
    fn set_name(&mut self, name: &[u8]) {
        let bytes = self.buf.to_mut();
        bytes.clear();
        bytes.extend(name);
        self.name_len = name.len();
    }

    fn clear_attributes(&mut self) {
        self.buf.to_mut().truncate(self.name_len);
    }
}