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/diagnostics/tests/j+k/SupertypeArgumentsNullability-SpecialTypes.kt

@@ -15,7 +15,6 @@ public class Y extends X<String> {
 // FILE: test.kt
 
 fun main() {
-    Y().foo().length
+    Y().foo().length()
     Y().bar(null)
 }
-