Generators

Add generators support to Spider.

```
fn inorder(node) {
if (node) {
yield* inorder(node.left);
yield t.label;
yield* inorder(node.right);
}
}
```

compiles to:

```
function* inorder(node) {
if (node) {
yield* inorder(node.left);
yield node.label;
yield* inorder(node.right);
}
}
```

Note that `*` is not required near the Spider function declaration.

If you have a better idea for the `yield*` syntax, please let me know.