Transform some builtin methods

1. CharSequence.charAt -> get
2. ML.remove(Int) -> removeAt,
3. MC.remove(Any?) -> MC.remove(E)

core/builtins/native/kotlin/CharSequence.kt

@@ -28,7 +28,7 @@ public interface CharSequence {
     /**
      * Returns the character at the specified [index] in the sequence.
      */
-    public fun charAt(index: Int): Char
+    public operator fun get(index: Int): Char
 
     /**
      * Returns a subsequence of this sequence.