Add missing header for AsyncFuncImpl

Summary: This source file uses symbols from async-func.h,
so it should actually include it!

Reviewed By: @paulbiss

Differential Revision: D1686540

Signature: t1:1686540:1416274384:94f6fe5dac421a8c5afa26a8a6f951abf6f87b74
Disable custom memcpy implementation on MacOSX

Summary: Mac's gcc doesn't understand this file.
Fall back on builtin memcpy until we can make it work properly.

Reviewed By: @paulbiss

Differential Revision: D1686529

Signature: t1:1686529:1416274291:b6ed1921ac0058c01a5b413d9c6a4069af50d59e
Use stderr for non-vital info when running benchy

Summary: Basically everything that benchy prints other than the final
table of results should be piped to stderr so that it can be filtered
out when running benchy as part of other scripts.

Reviewed By: @edwinsmith

Differential Revision: D1682346

Signature: t1:1682346:1416057424:7217a7242372904227108b26734423cd0292567f