Added benchmark for validation.

My machine was crazy loaded (almost entirely IO) while running this
test, but the relative time differences should be right.

BenchmarkCodeEncoder	      20	  66665200 ns/op	  29.11 MB/s
BenchmarkCodeMarshal	      50	  70372467 ns/op	  27.57 MB/s
BenchmarkCodeDecoder	      10	 163687732 ns/op	  11.85 MB/s
BenchmarkCodeUnmarshal	      10	 164917664 ns/op	  11.77 MB/s
BenchmarkCodeUnmarshalReuse	      10	 157644748 ns/op	  12.31 MB/s
BenchmarkValidation	     100	  20842076 ns/op	  93.10 MB/s
BenchmarkSkipValue	      50	  29496428 ns/op	  67.54 MB/s