Added _kdf_ all implementations and a simple and probably pointless test, also noted which functions are not available as they are not part of LazySodium

multiplatform-crypto-libsodium-bindings/src/commonMain/kotlin/com.ionspin.kotlin.crypto/kdf/Kdf.kt

@@ -0,0 +1,33 @@
+package com.ionspin.kotlin.crypto.kdf
+
+/**
+ * Created by Ugljesa Jovanovic (jovanovic.ugljesa@gmail.com) on 16/Sep/2020
+ */
+const val crypto_kdf_PRIMITIVE = "blake2b"
+const val crypto_kdf_BYTES_MIN = 16
+const val crypto_kdf_BYTES_MAX = 64
+const val crypto_kdf_CONTEXTBYTES = 8
+const val crypto_kdf_KEYBYTES = 32
+
+expect object Kdf {
+    /**
+     * The crypto_kdf_derive_from_key() function derives a subkey_id-th subkey subkey of length subkey_len bytes using
+     * the master key key and the context ctx.
+     * subkey_id can be any value up to (2^64)-1.
+     * subkey_len has to be between crypto_kdf_BYTES_MIN (inclusive) and crypto_kdf_BYTES_MAX (inclusive).
+     * Similar to a type, the context ctx is a 8 characters string describing what the key is going to be used for.
+     * Its purpose is to mitigate accidental bugs by separating domains. The same function used with the same key but
+     * in two distinct contexts is likely to generate two different outputs.
+     * Contexts don't have to be secret and can have a low entropy.
+     * Examples of contexts include UserName, __auth__, pictures and userdata.
+     * They must be crypto_kdf_CONTEXTBYTES bytes long.
+     * If more convenient, it is also fine to use a single global context for a whole application. This will still
+     * prevent the same keys from being mistakenly used by another application.
+     */
+    fun deriveFromKey(subkeyId: Int, subkeyLength: Int, context: String, masterKey: UByteArray) : UByteArray
+
+    /**
+     * The crypto_kdf_keygen() function creates a master key.
+     */
+    fun keygen() : UByteArray
+}