Add additional activation functions

[mewilhel]

• tanhshrink(x) = x - tanh(x) (concavoconvex)
• relu6(x) = min(max(0, x), 6) (convexoconcave)
• mish(x) = x * tanh(softplus(x)) (mixed convexity)
• lisht(x) = x * tanh(x)
• hardtanh(x) = max(-1, min(1, x)) (softmax)