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/codegen/boxWithStdlib/strings/kt5389_stringBuilderGet.kt

@@ -0,0 +1,4 @@
+fun box(): String {
+    val sb = StringBuilder("OK")
+    return "${sb.get(0)}${sb[1]}"
+}