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/properties/kt3556.kt

@@ -3,7 +3,7 @@ class Test {
     private val b : String get() = a
 
     fun outer() : Int {
-        return b.length
+        return b.length()
     }
 }