Tweaks to member-invoke traits
Tweaks to member-invoke traits

Summary:
[Folly] Tweaks to member-invoke traits.
* `const`.
* No need to delegate to `invoke`.

Reviewed By: aary

Differential Revision: D8632393

fbshipit-source-id: 08ae3bc6e3ede3766ec646a5d2588996ff734538

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly