Make CharSequence.length a function instead of property

And String.length as well.

This is done for JVM interoperability: java.lang.CharSequence is an open class
and has a function 'length()' which should be implemented in subclasses
somehow.

A minor unexpected effect of this is that String.length() is now a compile-time
constant (it wasn't such as a property because properties are not supported in
compile-time constant evaluation)

 #KT-3571 Fixed

compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -6286,6 +6286,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest(fileName);
         }
 
+        @TestMetadata("kt3571.kt")
+        public void testKt3571() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/strings/kt3571.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("kt3652.kt")
         public void testKt3652() throws Exception {
             String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/strings/kt3652.kt");