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/NestedLocalClass.kt

@@ -4,9 +4,8 @@ fun test(x: Any) {
   class LocalOuter {
     inner class Local {
       {
-        <!DEBUG_INFO_SMARTCAST!>x<!>.length
+        <!DEBUG_INFO_SMARTCAST!>x<!>.length()
       }
     }
   }
 }
-