make secure_memset function unsafe

[kitcatier]

rust-crypto/src/util.rs

Lines 32 to 39 in cc1a5fd

	pub fn secure_memset(dst: &mut [u8], val: u8) {
	unsafe {
	rust_crypto_util_secure_memset(
	dst.as_mut_ptr(),
	val,
	dst.len() as libc::size_t);
	}
	}

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.