hash: introduce memory-optimized merkle root hash calculation routine

NewMerkleTree is a memory hog, we can do better than that:

BenchmarkMerkle/NewMerkleTree-8                       13          88434670 ns/op        20828207 B/op     300035 allocs/op
BenchmarkMerkle/CalcMerkleRoot-8                      15          69264150 ns/op               0 B/op          0 allocs/op

pkg/core/helper_test.go

@@ -72,11 +72,7 @@ func newBlock(cfg config.ProtocolConfiguration, index uint32, prev util.Uint256,
 		},
 		Transactions: txs,
 	}
-	err := b.RebuildMerkleRoot()
-	if err != nil {
-		panic(err)
-	}
-
+	b.RebuildMerkleRoot()
 	b.Script.InvocationScript = testchain.Sign(b.GetSignedPart())
 	return b
 }