Add function to safely request the type-specific C++ element from a
C-style `TreeElement`. Looks like this:
```
TreeElementFoo *te_foo = tree_element_cast<TreeElementFoo>(te);
```
The "cast" will return null if the tree-element doesn't match the
requested type.
This is useful for the transition from the C-style type to the new ones.
08e2885796