* Path#ensureBlock keeps path context
This ensures that if you're inside an ArrowFunction with an expression body (say, you're on the BooleanLiteral in `() => true`), you don't suddenly lose your path context after inserting a variable.
This is because of 82d8aded8e (diff-9e0668ad44535be897b934e7077ecea5R14). Basically, an innocent `Scope#push` caused my visitor to suddenly stop working. Now, we mutate the Path so it's still in the tree.
* Tests
