add get_hint function to ChainingConf
add get_hint function to ChainingConf

Summary:
Rather than passing `Hint_t obj_t` directly as hint to `subexpressions` in
`handle_new_chain`, `handle_chaining` and `handle_continue_chain`, we
add a new callback to `ChainingConf` to allow us to process the `obj_t`
before passing it to `subexpression`.

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D39398256

fbshipit-source-id: 7f365106142fe095012d6376799c8856a06cd074

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