The order of attributes in an `Element` doesn't matter anymore.
`<elem a="b" c="d" />` and `<elem c="d" a="b" />` are now correctly
considered equal.
For that I had to derive `PartialOrd` and `Ord` for `Attribute`.
This allows us to sort cloned vectors of `Attribute` in the `PartialEq`
implementation and compare them instead of the struct `attributes`.
Fixes#3
0d0c4b76eb