Added _kx_ and nateive impl, updated libsodium submodule to latest commit

multiplatform-crypto-libsodium-bindings/src/commonMain/kotlin/com.ionspin.kotlin.crypto/keyexchange/KeyExchange.kt

@@ -0,0 +1,23 @@
+package com.ionspin.kotlin.crypto.keyexchange
+
+/**
+ * Created by Ugljesa Jovanovic
+ * ugljesa.jovanovic@ionspin.com
+ * on 09-Oct-2020
+ */
+
+const val crypto_kx_PUBLICKEYBYTES = 32
+const val crypto_kx_SECRETKEYBYTES = 32
+const val crypto_kx_SEEDBYTES = 32
+const val crypto_kx_SESSIONKEYBYTES = 32
+const val crypto_kx_PRIMITIVE = "x25519blake2b"
+
+data class KeyExchangeKeyPair(val publicKey: UByteArray, val secretKey: UByteArray)
+data class KeyExchangeSessionKeyPair(val receiveKey: UByteArray, val sendKey: UByteArray)
+
+expect object KeyExchange {
+    fun clientSessionKeys(clientPublicKey: UByteArray, clientSecretKey: UByteArray, serverPublicKey: UByteArray) : KeyExchangeSessionKeyPair
+    fun keypair() : KeyExchangeKeyPair
+    fun seedKeypair(seed: UByteArray) : KeyExchangeKeyPair
+    fun serverSessionKeys(serverPublicKey: UByteArray, serverSecretKey: UByteArray, clientPublicKey: UByteArray) : KeyExchangeSessionKeyPair
+}