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/LocalObject.kt

@@ -1,9 +1,9 @@
 fun foo(x: Any?) {
     if (x is String) {
         object : Base(<!DEBUG_INFO_SMARTCAST!>x<!>) {
-            fun bar() = <!DEBUG_INFO_SMARTCAST!>x<!>.length
+            fun bar() = <!DEBUG_INFO_SMARTCAST!>x<!>.length()
         }
     }
 }
 
-open class Base(s: String)
+open class Base(s: String)