Secure memory support

[Piotr Szturmaj]

We need a solution to store confidential data like encryption keys or 
passwords securely.

For example .NET has SecureString 
(http://msdn.microsoft.com/en-us/library/system.security.securestring.aspx).

At bare minimum program must be able to mark memory regions as non 
pageable, so private data will not be paged to disk.

On Windows there is VirtualLock/VirtualUnlock 
(http://msdn.microsoft.com/en-us/library/windows/desktop/aa366895%28v=vs.85%29.aspx). 
On *nixes there is mlock/munlock (http://linux.die.net/man/2/mlock).

Also, regions marked as secure must be zeroed after deallocation.

Should we make something like secure allocator in Phobos?