uint256: Add is odd 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
------------------------------------------------------------------
IsOdd   3.62ns ± 4%     1.64ns ± 1%     -54.65%  (p=0.000 n=10+10)

name    old allocs/op   new allocs/op   delta
---------------------------------------------------------
IsOdd   0.00            0.00            ~     (all equal)

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

internal/staging/primitives/uint256/uint256_bench_test.go

@@ -257,6 +257,34 @@ func BenchmarkBigIntIsZero(b *testing.B) {
 	}
 }
 
+// BenchmarkUint256IsOdd benchmarks determining if an unsigned 256-bit integer
+// is odd with the specialized type.
+func BenchmarkUint256IsOdd(b *testing.B) {
+	vals := randBenchVals
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i += len(vals) {
+		for j := 0; j < len(vals); j++ {
+			noElideBool = vals[j].n1.IsOdd()
+		}
+	}
+}
+
+// BenchmarkBigIntIsOdd benchmarks determining if an unsigned 256-bit integer is
+// odd with stdlib big integers.
+func BenchmarkBigIntIsOdd(b *testing.B) {
+	vals := randBenchVals
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i += len(vals) {
+		for j := 0; j < len(vals); j++ {
+			noElideBool = vals[j].bigN1.Bit(0) == 1
+		}
+	}
+}
+
 // BenchmarkUint256Eq benchmarks determining equality between two unsigned
 // 256-bit integers with the specialized type.
 func BenchmarkUint256Eq(b *testing.B) {