benchmark              old ns/op     new ns/op     delta
BenchmarkInit-4        3403          397           -88.33%
BenchmarkRead-4        14535         1560          -89.27%

* CTR-DRBG doesn't call "NewCipher" for block encryption
* Changes API of CTR-DRBG, so that read operation implementes io.Reader

Benchmark results:
----------------------
benchmark           old ns/op     new ns/op     delta
BenchmarkInit-4     1118          3579          +220.13%
BenchmarkRead-4     5343          14589         +173.05%

benchmark           old allocs     new allocs     delta
BenchmarkInit-4     15             0              -100.00%
BenchmarkRead-4     67             0              -100.00%

benchmark           old bytes     new bytes     delta
BenchmarkInit-4     1824          0             -100.00%
BenchmarkRead-4     9488          0             -100.00%