pub enum NodeData {
    Element(Element),
    Text(NodeChars),
}

The old NodeData is now Element.

This will let us box the Element, so that it takes as little space as
possible for the enum when it has a NodeData::Text value.

To make this easier for the rest of the code, now there is a
NodeBorrow trait, implemented for RsvgNode and imported pretty much
everywhere, with the following methods:

pub trait NodeBorrow {
    /// Returns `false` for NodeData::Text, `true` otherwise.
    fn is_element(&self) -> bool;

    /// Borrows a `NodeChars` reference.
    ///
    /// Panics: will panic if `&self` is not a `NodeData::Text` node
    fn borrow_chars(&self) -> Ref<NodeChars>;

    /// Borrows an `Element` reference
    ///
    /// Panics: will panic if `&self` is not a `NodeData::Element` node
    fn borrow_element(&self) -> Ref<Element>;

    /// Borrows an `Element` reference mutably
    ///
    /// Panics: will panic if `&self` is not a `NodeData::Element` node
    fn borrow_element_mut(&mut self) -> RefMut<Element>;
}

So, instead of doing node.borrow() -> Ref<NodeData>, we now can do
node.borrow_element() directly, for example.

The calling code is still supposed to first ensure that a node
is_element() or not, just like it was doing
`get_type() == NodeType::Foo` before doing get_impl::<Foo>().