Performance worse in simple scenarios

[chuyskywalker]

I was looking into adding ffjson to (https://github.com/alecthomas/go_serialization_benchmarks (like #154 was suggesting) and found that ffjson made things a good deal worse in the benchmark.

https://github.com/chuyskywalker/jsontest

On my local machine, with struct_ffjson.go removed:

BenchmarkJsonMarshal-8         500000     2475 ns/op       536 B/op       6 allocs/op
BenchmarkJsonUnmarshal-8       500000     3452 ns/op       447 B/op       8 allocs/op

With struct_ffjson.go in place:

BenchmarkJsonMarshal-8         500000     3495 ns/op       949 B/op      13 allocs/op
BenchmarkJsonUnmarshal-8       300000     4350 ns/op       848 B/op      14 allocs/op

Maybe I did something wrong here?