sigma function implemented incorrectly

[themighty1]

Currently, the sigma function is implemented as per this comment

swanky/scuttlebutt/src/hash_aes.rs

Line 44 in bf632db

/// function and `σ(x₀ || x₁) = (x₀ ⊕ x₁) || x₁`.

which is not correct. It should be σ(x₀ || x₁) = (x₀ ⊕ x₁) || x₀ <-- the second half must be replaced by the first half.