Add a NodeData::get_chars() function
Add a NodeData::get_chars() function

We'll be changing the internal representation of the "chars" node
vs. other real SVG element types.  So, instead of the usual
.get_impl::<NodeChars>() we'll have this explicit function.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg