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

compiler/testData/diagnostics/tests/j+k/kt1730_implementCharSequence.kt

@@ -0,0 +1,24 @@
+// KT-1730 Method which has been implemented by Java is recognized to be abstract.
+
+// FILE: C.java
+public class C implements java.lang.CharSequence {
+    @Override
+    public int length() {
+        return 3;
+    }
+    @Override
+    public char charAt(int index) {
+        return 48;
+    }
+    @Override
+    public CharSequence subSequence(int start, int end) {
+        return "ab";
+    }
+    @Override
+    public String toString() {
+        return "abc";
+    }
+}
+
+// FILE: T.kt
+class T : C()