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
 }