uint256: Add squaring benchmarks.

The following is a comparison between stdlib big integers (old) and the
specialized type (new) averaging 10 runs each:

name     old time/op     new time/op     delta
-------------------------------------------------------------------
Square   418ns ± 0%      7ns ± 2%        -98.39%  (p=0.000 n=10+10)

name     old allocs/op   new allocs/op   delta
--------------------------------------------------------------------
Square   1.00 ± 0%       0.00            -100.00%  (p=0.000 n=10+10)

This is part of a series of commits to fully implement the uint256
package.

internal/staging/primitives/uint256/uint256_bench_test.go

@@ -616,3 +616,37 @@ func BenchmarkBigIntMulUint64(b *testing.B) {
 		}
 	}
 }
+
+// BenchmarkUint256Square benchmarks computing the quotient of unsigned 256-bit
+// integers with the specialized type.
+func BenchmarkUint256Square(b *testing.B) {
+	n := new(Uint256)
+	vals := randBenchVals
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i += len(vals) {
+		for j := 0; j < len(vals); j++ {
+			val := &vals[j]
+			n.SquareVal(val.n1)
+		}
+	}
+}
+
+// BenchmarkBigIntSquare benchmarks computing the quotient of unsigned 256-bit
+// integers with stdlib big integers.
+func BenchmarkBigIntSquare(b *testing.B) {
+	n := new(big.Int)
+	two256 := new(big.Int).Lsh(big.NewInt(1), 256)
+	vals := randBenchVals
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i += len(vals) {
+		for j := 0; j < len(vals); j++ {
+			val := &vals[j]
+			n.Mul(val.bigN1, val.bigN1)
+			n.Mod(n, two256)
+		}
+	}
+}