make secure_memset function unsafe

[kitcatier]

https://github.com/DaGenix/rust-crypto/blob/cc1a5fde1ce957bd1a8a2e30169443cdb4780111/src/util.rs#L32-L39 Hello, if a function's entire body is unsafe, the function is itself unsafe and should be marked appropriately, which will make the caller ignore the safety requirements that the function parameters must guarantee, the developer who calls the secure_memset function may not notice this safety requirement.

Marking them unsafe also means that callers must make sure they know what they're doing.