Wrap randombytes

[silkeh]

This wraps randombytes as it is in libsodium 1.0.12.

There are two things to note:

• The RandomBytes() function does not place bytes in a buffer like in libsodium, but returns a specified number of random bytes instead. Example:

  randBytes := randombytes.RandomBytes(16)

  Perhaps it should be named RandomBytesBytes().
• RandomBytesSalsa20Implementation and RandomBytesSysRandomImplementation are pointers, making the use of RandomBytesSetImplementation() as follows:

  randombytes.RandomBytesSetImplementation(randombytes.RandomBytesSalsa20Implementation)

[redragonx]

It would be useful if there was a comment that mentions the different expected outputs (with the changes)

[silkeh]

For completeness I have added comments to all functions explaining their use.

[redragonx]

Thanks for doing that!