Remove transmutes, use a pointer cast and fail on BE

[hdevalence]

Because the keccak crate takes a state as [u64; 25] (cf RustCrypto/sponges#3), the code currently does a transmute to convert from bytes.

This should be replaced with a pointer cast on little-endian machines.

On big-endian machines, a byteswap is probably necessary, but since I don't have one to test on and I'm not sure that the keccak crate works correctly there, it seems better to just fail to compile rather than provide possibly-wrong code.

[hdevalence]

Closed by #20