stdlib: Improve character API.

The patch adds methods for surrogate characters processing and Unicode
category support needed for regular expression library.

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

@@ -1,6 +1,6 @@
 fun main(args : Array<String>) {
     val hello = "Hello"
-    val array = toCharArray(hello)
+    val array = hello.toCharArray()
     for (ch in array) {
         print(ch)
         print(" ")