Use std::copy in serial/TBB Copy implementation.

I had assumed that the compiler would be clever enough to turn the iterative implementation of Copy into a memcpy, but inspecting the disassembly on a release GCC build shows that this is not the case, likely because it can't assume that the memory ranges do not overlap.

Replacing the loop with std::copy speeds things up (about 30-50%) for most data types, though there is a slight (usually < 5%) slowdown for Vec types. The uint8 copy improved by a factor of 7.

Comparison:

Speedup	iteration	std::copy	Benchmark (Type)
1.363	0.001590 +- 0.000087	0.001166 +- 0.000049	Copy 2097152 values (vtkm::Float32)
1.487	0.003429 +- 0.000185	0.002305 +- 0.000146	Copy 2097152 values (vtkm::Float64)
1.379	0.001568 +- 0.000072	0.001137 +- 0.000093	Copy 2097152 values (vtkm::Int32)
1.420	0.003410 +- 0.000173	0.002402 +- 0.000101	Copy 2097152 values (vtkm::Int64)
1.303	0.001564 +- 0.000083	0.001201 +- 0.000078	Copy 2097152 values (vtkm::UInt32)
7.204	0.002441 +- 0.000104	0.000339 +- 0.000029	Copy 2097152 values (vtkm::UInt8)
0.987	0.006602 +- 0.000266	0.006688 +- 0.000291	Copy 2097152 values (vtkm::Vec< vtkm::Float32, 4 >)
0.965	0.010065 +- 0.000528	0.010427 +- 0.000617	Copy 2097152 values (vtkm::Vec< vtkm::Float64, 3 >)
0.979	0.003327 +- 0.000191	0.003398 +- 0.000142	Copy 2097152 values (vtkm::Vec< vtkm::Int32, 2 >)
0.851	0.001579 +- 0.000090	0.001856 +- 0.000098	Copy 2097152 values (vtkm::Vec< vtkm::UInt8, 4 >)