Update next version to 0.12.0.

It will most likely not be backwards-compatible.
Remove WeightedWalk.

It turns out this was a bad way to model the problem, because it makes
sharing predecessor walks difficult. Walk suffers from the same problem,
so it'll be the next class to go.
Add PersistentList.last().

This is simple enough to iterate and find, but it needs to be efficient
because this will be replacing the use case for Walk.getFrom().
Add back PersistentList.size() method.
Remove unused imports.
Move Walk.Step to a top-level abstract class, with two factory methods.
Remove Walk.

Replaced the Walk usage in elements/leafElements/elementPath with a
PersistentList<Step<Object,String>>.

This is now the preferred idiom for tracking walks, a PersistentList<T>,
where T can be anything, but is most likely some variant of Step.
Remove now unused PersistentList.addAll(Iterable) method.
Add Step factory method for when there is no edge.