make BaseTraversal and ConstBaseTraversal template classes

We have an upcoming need to rewrite parts of the AST (overwrite one AST node
with another). This would be easier done if we could return a new Node from a
visitation function. To achieve this we need a tweakable return type as part of
the visitor base classes.

Note that we cannot make the intermediate children of BaseTraversal and
ConstBaseTraversal (e.g. Traversal) also template classes, because there is no
straightforward way to construct a value to return when the template type is
instantiated as non-void. We could play some tricks with template specialisation
but it seemed easier to just leave them as concrete classes.