In Attributes, store the values long-term as interned atoms
In Attributes, store the values long-term as interned atoms

Previously we just stored &str references with the lifetime of the
libxml2 call to the start_element callback.  Now our Attributes is
independent, as it stores a copy of the values.

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