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/dataFlow/local/LocalClassProperty.kt

@@ -2,11 +2,11 @@ fun test(x: Any?) {
   if (x !is String) return
 
   class C {
-    val v = <!DEBUG_INFO_SMARTCAST!>x<!>.length
+    val v = <!DEBUG_INFO_SMARTCAST!>x<!>.length()
 
     val vGet: Int
-      get() = <!DEBUG_INFO_SMARTCAST!>x<!>.length
+      get() = <!DEBUG_INFO_SMARTCAST!>x<!>.length()
 
     val s: String = <!DEBUG_INFO_SMARTCAST!>x<!>
   }
-}
+}