Securely zero memory while avoiding compiler optimizations:
unified 'secure_zero_memory()' wrapper for secure intrinsic
functions for zeroing memory, using FFI to invoke OS intrinsics
on stable (with support for Linux, Windows, OS X/iOS, FreeBSD,
OpenBSD, NetBSD, DragonflyBSD), or the unstable
'volatile_set_memory()` intrinsic on nightly.
No insecure fallbacks, no dependencies, no std, no functionality
besides securely zeroing memory.