core: reduce big.Int copies

[jmank88]

pprof memprofile for BenchmarkStateProcessor_Process showed (*stateObject).Setbalance accounting for 4% of alloc_space and 9.7% of alloc_objects:

190.01MB  4.06% 48.68%   291.52MB  6.23%  github.com/gochain-io/gochain/core/state.(*stateObject).SetBalance

5869752  9.70% 22.57%    7936219 13.12%  github.com/gochain-io/gochain/core/state.(*stateObject).SetBalance

The big.Int copy responsible for the allocations isn't necessary, since the copied field is set in the method call on the next line.
Here are benchcmp results after removing the copy:

benchmark                                    old ns/op      new ns/op      delta
BenchmarkStateProcessor_Process/____1-4      75714          71901          -5.04%
BenchmarkStateProcessor_Process/___10-4      188055         183828         -2.25%
BenchmarkStateProcessor_Process/__100-4      1313477        1298224        -1.16%
BenchmarkStateProcessor_Process/_1000-4      12085332       12429412       +2.85%
BenchmarkStateProcessor_Process/_10000-4     1564422395     1135580047     -27.41%

benchmark                                    old allocs     new allocs     delta
BenchmarkStateProcessor_Process/____1-4      253            244            -3.56%
BenchmarkStateProcessor_Process/___10-4      1243           1144           -7.96%
BenchmarkStateProcessor_Process/__100-4      11143          10144          -8.97%
BenchmarkStateProcessor_Process/_1000-4      110145         102152         -7.26%
BenchmarkStateProcessor_Process/_10000-4     1462026        1369251        -6.35%

benchmark                                    old bytes     new bytes     delta
BenchmarkStateProcessor_Process/____1-4      44579         44225         -0.79%
BenchmarkStateProcessor_Process/___10-4      93326         89372         -4.24%
BenchmarkStateProcessor_Process/__100-4      580901        540950        -6.88%
BenchmarkStateProcessor_Process/_1000-4      5456650       5216035       -4.41%
BenchmarkStateProcessor_Process/_10000-4     79498328      76030176      -4.36%