Rename CharSequence.get to charAt

This is done for JVM interoperability. There's still a member function
String.get() and an extension function "get" on CharSequence

 #KT-1730 Fixed
 #KT-5389 Fixed

core/builtins/native/kotlin/CharSequence.kt

@@ -17,7 +17,7 @@
 package kotlin
 
 public trait CharSequence {
-    public fun get(index: Int): Char
-
     public fun length(): Int
+
+    public fun charAt(index: Int): Char
 }

core/builtins/native/kotlin/String.kt

@@ -19,9 +19,11 @@ 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 fun get(index: Int): Char
 
     public override fun length(): Int
+
+    public override fun charAt(index: Int): Char
+
+    public override fun compareTo(other: String): Int
 }