tests: Add benchmarks for Decode and Encode

- use existing tests vector as input data
- for Decode we prepare one large pickle stream from testv and decode all pickle from there
- for Encode we prepare one lirge slice with all objects from testv and encode it

On an i7-6600U currently it looks like:

    BenchmarkSpeed-4         5000000               387 ns/op             280 B/op          8 allocs/op
    BenchmarkDecode-4          20000             81892 ns/op           43989 B/op        502 allocs/op
    BenchmarkEncode-4         100000             16711 ns/op            6536 B/op        297 allocs/op