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/intrinsics/stringFromCollection.kt

@@ -2,6 +2,6 @@ fun box(): String {
     val list = java.util.ArrayList<String>()
     list.add("0")
     list[0][0]
-    list[0].length
+    list[0].length()
     return "OK"
 }