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/safeCall/kt247.kt

@@ -1,7 +1,7 @@
 fun t1() : Boolean {
     val s1 : String? = "sff"
     val s2 : String? = null
-    return s1?.length == 3 && s2?.length == null
+    return s1?.length() == 3 && s2?.length() == null
 }
 
 fun t2() : Boolean {