Visitor<T> instead of Visitor

[voiser]

Hi,

In order to perform AST transformations, like filtering or transcompiling, the Visitor interface could be redefined like:

interface Visitor<T> {
   ...
   T visit(BlahBlahNode node);
   ...
}

And the Node interface should offer something like:

interface Node {
    T accept(Visitor<T> v)
}

That would let us do something like:

MyASTNode transformed = rootNode.visit(new Visitor<MyASTNode>  {...})

Current visitors could be declared as:

class ... implements Visitor<Void>

I think this change would have a low impact in the code.

What do you think?