(Wangle) Allocate lambda space inside Core instead of inside std::function

(Wangle) Allocate lambda space inside Core instead of inside std::function

Summary:
Taking this trick that is used in the fibers library. We can keep 64
bytes of space inside `Core` to allocate the callback lambda into,
instead of having `std::function` do another `malloc`. This seems to
greatly improve the synthethic benchmark, and hopefully production
workloads, too, by reducing the number of mallocs.

64 bytes were picked because it fits all the lambdas in the futures
tests. We might want to adjust this based on production data...?

https://fb.intern.facebook.com/groups/715931878455430/permalink/837898842925399/

Test Plan: Run all the tests for all platforms, compilers, and Windtunnel.

Reviewed By: hans@fb.com

Subscribers: chalfant, meisner, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1931620

Signature: t1:1931620:1427841595:6ec667b58980be232dfb116bc316148bb67de4fc