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/classes/kt940.kt

@@ -4,8 +4,8 @@ fun box() : String {
     val w = object : Comparator<String?> {
 
         override fun compare(o1 : String?, o2 : String?) : Int {
-            val l1 : Int = o1?.length ?: 0
-            val l2 = o2?.length ?: 0
+            val l1 : Int = o1?.length() ?: 0
+            val l2 = o2?.length() ?: 0
             return l1 - l2
         }