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

js/js.libraries/src/core/string.kt

@@ -27,8 +27,6 @@ public fun String.split(regex: String, limit: Int): Array<String> = noImpl
 native public fun String.substring(beginIndex : Int) : String = noImpl
 native public fun String.substring(beginIndex : Int, endIndex : Int) : String = noImpl
 
-native public fun String.charAt(index : Int) : Char = noImpl
-
 native public fun String.concat(str : String) : String = noImpl
 
 native public fun String.match(regex : String) : Array<String> = noImpl

js/js.translator/src/org/jetbrains/k2js/translate/intrinsic/functions/factories/StringOperationFIF.java

@@ -26,7 +26,9 @@ public final class StringOperationFIF extends CompositeFIF {
     public static final FunctionIntrinsicFactory INSTANCE = new StringOperationFIF();
 
     private StringOperationFIF() {
-        add(pattern("kotlin", "CharSequence", "get").checkOverridden(), new BuiltInFunctionIntrinsic("charAt"));
+        add(pattern("kotlin", "String", "get"), new BuiltInFunctionIntrinsic("charAt"));
+        // This intrinsic is needed because charAt is a public function taking one parameter and thus its name would be mangled otherwise
+        add(pattern("kotlin", "CharSequence", "charAt").checkOverridden(), new BuiltInFunctionIntrinsic("charAt"));
         add(pattern("kotlin", "CharSequence", "length").checkOverridden(), LENGTH_PROPERTY_INTRINSIC);
         add(pattern("kotlin.js", "isEmpty").isExtensionOf("kotlin.CharSequence"), IS_EMPTY_INTRINSIC);
     }