add general support from encoding to string and restoring from string various keys formats. + docs.

src/commonTest/kotlin/KeysTest.kt

@@ -416,4 +416,45 @@ class KeysTest {
         assertEquals(x, BipackDecoder.decode<SecretKey>(y))
         assertContentEquals(x.keyBytes, BipackDecoder.decode<SecretKey>(y).keyBytes)
     }
+
+    @Test
+    fun testStringRepresentationAndParse() = runTest {
+        initCrypto()
+        val k1 = SigningSecretKey.new()
+        val k2 = k1.verifyingKey
+        val k3 = SecretKey.new()
+        val k4 = k3.publicKey
+
+        val k5 = UniversalPrivateKey.new()
+        val k6 = k5.publicKey
+
+        assertEquals(32, k2.keyBytes.size)
+        assertContentEquals(k2.keyBytes, k2.id.id.body)
+
+        val k7 = SymmetricKey.new()
+        val k8 = KDF.Complexity.Interactive.derive("super")
+
+        fun testToString(k: UniversalKey) {
+            val s = k.toString()
+            val kx = UniversalKey.parseString(s)
+            assertEquals(kx::class, k::class)
+            assertContentEquals(k.keyBytes, kx.keyBytes)
+            assertEquals(k.id, kx.id)
+            assertEquals(k, kx)
+        }
+
+        fun testAsString(k: UniversalKey) {
+            val s = k.asString()
+            val kx = UniversalKey.parseString(s)
+            assertEquals(kx::class, k::class)
+            assertContentEquals(k.keyBytes, kx.keyBytes)
+            assertEquals(k.id, kx.id)
+            assertEquals(k, kx)
+        }
+
+        testToString(k2)
+        testToString(k4)
+
+        for( i in listOf(k1, k2, k3, k4, k5, k6, k7, k8)) testAsString(i)
+    }
 }