Split Library.kt into several files

For better readbility and discoverability. Also rename String.compareTo
parameter to "other", since that is the name of parameter of other compareTo's

core/builtins/native/kotlin/String.kt

@@ -0,0 +1,9 @@
+package kotlin
+
+public class String : Comparable<String>, CharSequence {
+    public fun plus(other: Any?): String
+
+    public override fun compareTo(other: String): Int
+    public override fun get(index: Int): Char
+    public override val length: Int
+}