[feature] implement a visitor & filter helper

implement a visitor helper in order to handle AST transversal :

```js
const ast = parser.parse('...some code...');

// visiting child nodes :
ast.visit(function(node) {
// do something
});

// filtering a list of nodes
const nodes = ast.filter(function(node) {
return node.kind === 'something';
});
```

> NOTE : this is implemented on each node, can be used on program node as in the example, or on any other node