seckey

Use memsec protected secret memory.

exmaple

```rust use seckey::SecKey;

let secpass = SecKey::new([8u8; 8]).unwrap();

{ assert_eq!([8u8; 8], *secpass.read()); }

{ let mut wpass = secpass.write(); wpass[0] = 0; assert_eq!([0, 8, 8, 8, 8, 8, 8, 8], *wpass); } ```