Fix pointer alignment

[hdevalence]

The keccakf crate doesn't operate on a state composed of bytes, it operates on a state composed of words. (cf #11, #20). This is currently handled by doing

fn transmute_state(st: &mut [u8; 200]) -> &mut [u64; 25] {
    unsafe { &mut *(st as *mut [u8; 200] as *mut [u64; 25]) }
}

Unfortunately this isn't quite right because the second pointer has tighter alignment requirements than the first pointer, which isn't guaranteed to have u64 alignment.

[hdevalence]

Storing the state as words is unappealing because it's a) wrong -- the state is a byte array and b) it means that we have to do a conversion on every byte access. This might be fine on LE systems where the conversion is a no-op but it seems bad on a BE system, where it would be preferable to do a byteswap before calling keccakf and then swap back.

[hdevalence]

@myrrlyn suggests getting the correct alignment by wrapping the bytes in a newtype with #[repr(align(64))]