uint256: optimize AddMod (#165)

```
AddMod/small/uint256-8   21.0ns ±44%  17.0ns ±43%     ~     (p=0.052 n=10+10)
AddMod/mod64/uint256-8   28.6ns ±36%  26.6ns ±44%     ~     (p=0.481 n=10+10)
AddMod/mod128/uint256-8  56.1ns ±43%  53.7ns ±49%     ~     (p=0.810 n=10+10)
AddMod/mod192/uint256-8  62.0ns ±44%  62.2ns ±44%     ~     (p=0.971 n=10+10)
AddMod/mod256/uint256-8  25.4ns ±25%  12.0ns ±44%  -52.74%  (p=0.000 n=10+9)
```

conversion_test.go

@@ -401,7 +401,7 @@ func BenchmarkSetBytes(b *testing.B) {
 			val.SetBytes(bytearr[:27])
 			val.SetBytes(bytearr[:28])
 			val.SetBytes(bytearr[:29])
-			val.SetBytes(bytearr[:20])
+			val.SetBytes(bytearr[:30])
 			val.SetBytes(bytearr[:31])
 			val.SetBytes(bytearr[:32])
 		}

uint256.go

@@ -244,12 +244,10 @@ func (z *Int) AddMod(x, y, m *Int) *Int {
 
 		// final sub was unnecessary
 		if c1 == 0 && c2 != 0 {
-			copy((*z)[:], res[:])
-			return z
+			return z.Set(&res)
 		}
 
-		copy((*z)[:], tmp[:])
-		return z
+		return z.Set(&tmp)
 	}
 
 	if m.IsZero() {