primitives: Add check proof of work 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
-----------------------------------------------------------------------------
CheckProofOfWork   314ns ± 1%      43ns ± 0%       -86.42%  (p=0.000 n=10+10)

name               old allocs/op   new allocs/op   delta
------------------------------------------------------------------------------
CheckProofOfWork   5.00 ± 0%       0.00            -100.00%  (p=0.000 n=10+10)

internal/staging/primitives/pow_bench_test.go

@@ -58,3 +58,22 @@ func BenchmarkHashToUint256(b *testing.B) {
 		HashToUint256(hash)
 	}
 }
+
+// BenchmarkCheckProofOfWork benchmarks ensuring a given block hash satisfies
+// the proof of work requirements for given difficulty bits.
+func BenchmarkCheckProofOfWork(b *testing.B) {
+	// Data from block 100k on the main network.
+	h := "00000000000004289d9a7b0f7a332fb60a1c221faae89a107ce3ab93eead2f93"
+	blockHash, err := chainhash.NewHashFromStr(h)
+	if err != nil {
+		b.Fatalf("unexpected error: %v", err)
+	}
+	const diffBits = 0x1a1194b4
+	powLimit := hexToUint256(mockMainNetPowLimit())
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = CheckProofOfWork(blockHash, diffBits, powLimit)
+	}
+}