folly::fibers::Fiber: use folly::Function instead of std::function

folly::fibers::Fiber: use folly::Function instead of std::function

Summary:We are jumping through some ugly hoops in the implementation of
fibers to allow for non-copyable functors/lambdas. We can get rid of
those by using folly::Function instead of std::function to store
functions in folly::fibers::Fiber.

This involves one observable interface change: the virtual function
folly::fibers::InlineFunctionRunner::run must take a
folly::Function<void()> argument instead of a std::function<void()>.

Reviewed By: andriigrynenko

Differential Revision: D3102711

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fbshipit-source-id: 56705b972dc24cc0da551109ed44732b97cb6e13