x/crypto/salsa20: implement cipher.Stream interface

[FiloSottile]

salsa20 is the odd one out that implements only a stateless XORKeyStream at the package level.

[as]

The nonce is currently exposed to the user, how would the cipher.Stream interface modify the nonce? simple increment?

How does the constructor handle the semantics for the different salsa lengths?

[jannson]

I implement an example here:
https://github.com/jannson/salsa20stream

You can read usage in example:
ver1: https://github.com/jannson/salsa20stream/blob/master/salsa20stream_test.go#L15
ver2: https://github.com/jannson/salsa20stream/blob/master/salsa20stream_test.go#L29

bench comparing with aes also in tests.