make Flow_ast_utils.fold_bindings_* support typed asts
make Flow_ast_utils.fold_bindings_* support typed asts

Summary: Changelog: [internal]

Reviewed By: panagosg7

Differential Revision: D31475741

fbshipit-source-id: 0bb73fe186c8750590eb18636d88fd6887d79e65

facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow