(Alpha quality preview) 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.