folly: ubsan: replace undefined call through reinterpret-cast fn-pointer with std...

folly: ubsan: replace undefined call through reinterpret-cast fn-pointer with std::function

Summary:
This code casts function pointers to void(*fn)(void*) and calls
functions through that type. Calling functions through a different
pointer type is undefined behavior. Casts were used to avoid memory
allocations.

`std::bind` needs a memory allocation - so it's avoided in EventBase.

  std::function<void()> x = std::bind(fn, args);

`std::function` should use small object optimizations when possible and embed the functor in the body of `std::function`.

On GCC 5.0 and forward small lambdas don't need memory allocations
(lambdas being tiny structures - two pointers in this case - and a
function operator).

  std::function<void()> y = [=] { fn(args); };

On GCC 4.7 .. 4.9 functors for which __is_location_invariant<Func> is
true also don't need memory allocations.

Remove undefined behavior by using a `SmallFunctor` which leverages `std::function`'s small object optimization.

Reviewed By: philippv

Differential Revision: D2864895

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