Summary:This is an ugly but occassionally useful hack to accomodate cases
where we want to propagate folly::Function but run into the brick wall that is
std::function

@override-unit-failures

Reviewed By: spacedentist

Differential Revision: D3118432

fb-gh-sync-id: 80ed56494dfcf60e0f266013d880107acabc7dcf
fbshipit-source-id: 80ed56494dfcf60e0f266013d880107acabc7dcf