Function g_first_kernel 

pub fn g_first_kernel(input: GState, message: b32) -> GState

Synthesizable first half of G: rotations by 16 and 12 bits.

Inputs: the four words and one 32-bit message word. Output: the four updated words. Combinational: a 3-input add, then an XOR, then an add, then an XOR. Rotations are pure wiring.