benchmark struct of struct of int64

benchmark encoding and decoding a struct of struct of int64.

perfs before c8dd3d1 (ie: b46c98d)
```
BenchmarkEncodeStructI1	 5000000	       689 ns/op
BenchmarkEncodeStructI2	 5000000	       372 ns/op
BenchmarkEncodeStructI3	 5000000	       365 ns/op
BenchmarkDecodeStructI1	 2000000	       807 ns/op
BenchmarkDecodeStructI2	 2000000	       805 ns/op
BenchmarkDecodeStructI3	 2000000	       811 ns/op
```

perfs after c8dd3d1 (ie: ad72c3c)
```
BenchmarkEncodeStructI1	 1000000	      1107 ns/op
BenchmarkEncodeStructI2	 2000000	       782 ns/op
BenchmarkEncodeStructI3	 2000000	       786 ns/op
BenchmarkDecodeStructI1	 2000000	       803 ns/op
BenchmarkDecodeStructI2	 2000000	       849 ns/op
BenchmarkDecodeStructI3	 2000000	       801 ns/op
```

encoding of course takes way more time (as before c8dd3d1 the embedded struct was dropped on the floor)

for comparison, here is the before/after for a simple struct holding one int64:

before
```
BenchmarkEncodeStructI1	 2000000	       808 ns/op
BenchmarkEncodeStructI2	 5000000	       516 ns/op
BenchmarkEncodeStructI3	 5000000	       514 ns/op
BenchmarkDecodeStructI1	 2000000	       862 ns/op
BenchmarkDecodeStructI2	 5000000	       531 ns/op
BenchmarkDecodeStructI3	 5000000	       510 ns/op
```

after
```
BenchmarkEncodeStructI1	 2000000	       789 ns/op
BenchmarkEncodeStructI2	 5000000	       493 ns/op
BenchmarkEncodeStructI3	 5000000	       496 ns/op
BenchmarkDecodeStructI1	 2000000	       834 ns/op
BenchmarkDecodeStructI2	 5000000	       512 ns/op
BenchmarkDecodeStructI3	 5000000	       487 ns/op
```

everything is in the noise.