primitives: Add work calc benchmark.

The following is a comparison between the existing implementation in the
blockchain/standalone module with big integers (old) and the new
implementation (new) averaging 10 runs each:

name       old time/op     new time/op     delta
---------------------------------------------------------------------
CalcWork   638ns ± 1%      59ns ± 1%       -90.78%  (p=0.000 n=10+10)

name       old allocs/op   new allocs/op   delta
----------------------------------------------------------------------
CalcWork   7.00 ± 0%       0.00            -100.00%  (p=0.000 n=10+10)

internal/staging/primitives/pow_bench_test.go

@@ -30,3 +30,13 @@ func BenchmarkUint256ToDiffBits(b *testing.B) {
 		Uint256ToDiffBits(n)
 	}
 }
+
+// BenchmarkCalcWork benchmarks calculating a work value from difficulty bits.
+func BenchmarkCalcWork(b *testing.B) {
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		const input = 0x1b01330e
+		CalcWork(input)
+	}
+}