Implement Iterable for ast.Node instead of a hand-crafted Traversable

[fpoli]

As @marcoeilers pointed out, PR #493 removed too many methods from the interface. Manually re-implementing all of them in the utils.Traversable trait is a lot of work. This PR provides a different trade-off between performance and usability.

Pro:

• ast.Node extends again the Iterable trait, so it inherits all its convenient methods.
• foreach just calls the visitor and is thus efficient.
• Compared to 1df0f34, iterator's implementation just allocates one collection containing all the AST's subnodes, rather than doing it for each node of the AST.

Cons:

• All methods inherited from ast.Node will internally call iterator, thus they'll create a collection containing all the AST's subnodes. In most of the cases, the allocation can be avoided by overriding the method such that it internally uses foreach instead of iterator.

[mschwerhoff]

@fpoli If we bake in the assumption that all nodes are immutable, we could turn subnodes and iterator (and maybe other defs) into lazy vals. This could speed up subsequent traversals of the same tree.