3-bit ripple-carry adder

Chain three full adders into a 3-bit ripple-carry adder that computes a + b + cin.

Unlike a plain adder, this problem exposes the internal carry chain: cout[2:0] must contain the carry-out of each full adder stage. Bit cout[0] is the carry out of the least-significant stage, cout[1] of the middle stage, and cout[2] of the most-significant stage (this last one is the usual final carry-out).