Modify voice to use encryption instead of plain.

[bwmarrin]

Discord plans to drop support for non-encrypted voice. So, we need to get voice encryption support added quickly.

Discord is using NaCl SecretBox for encryption (https://nacl.cr.yp.to/secretbox.html)

• https://godoc.org/golang.org/x/crypto/nacl/secretbox
• https://github.com/andmarios/golang-nacl-secretbox

[bwmarrin]

Rough steps for encryption.

Step 1

Need to pass xsalsa20_poly1305 as the mode for voiceUDPOp{} in voice.go:udpOpen() func. to get the encryption party started.
85e06e3#diff-952a4895d262bd9f63ab49444784519dR398

Step 2

Then, on the WS opcode 4 comes back that should contain the encryption secret

Example op 4

{
        "secret_key": [
                254,
                232,
                117,
.....  this continues to create a total of 32 int values
                180,
                232
        ],
        "mode": "xsalsa20_poly1305"
}

85e06e3#diff-952a4895d262bd9f63ab49444784519dR205

Step -- Send Audio

After we build the packet to send, encrypt the opus data before sending
85e06e3#diff-952a4895d262bd9f63ab49444784519dR494

Step -- Receive Audio

When we receive a packet, decrypt the opus data
85e06e3#diff-952a4895d262bd9f63ab49444784519dR574