Report back if any changes was made to AST. Fixes #152

This is done by using the proposed ast-types API from: benjamn/ast-types#79
Now that we reset `visitor._changeReported` every time we call `visitor.visit` (or `recast.visit`), I think you should be able to move the visitor creation back outside this function, so that it doesn't have to happen every time we call `regenerator.transform`.