String <-> CharArray conversion, ArrayBuilder by @elizarov (#98)

backend.native/tests/runtime/basic/tostring2.kt

@@ -0,0 +1,10 @@
+fun main(args : Array<String>) {
+    val hello = "Hello"
+    val array = toCharArray(hello)
+    for (ch in array) {
+        print(ch)
+        print(" ")
+    }
+    println()
+    println(fromCharArray(array, 0, array.size))
+}