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/testData/codegen/box/super/kt4982.kt

@@ -13,9 +13,9 @@ fun box(): String {
 
         override fun condition(): Boolean {
             result = "OK"
-            return result.length == 2
+            return result.length() == 2
         }
     }
 
     return result;
-}
+}