update reader and implement class field

lib/bosl/compiler/class_field.rb

@@ -0,0 +1,22 @@
+module Bosl
+  Compiler.class_eval do
+
+    def on_class_field expression
+      puts expression.inspect
+      type , name , value = *expression
+
+      for_class = self.method.for_class
+      index = for_class.object_layout.variable_index(name)
+      raise "class field already defined:#{name} for class #{for_class.name}" if index
+      puts "Define field #{name} on class #{for_class.name}"
+      index = for_class.object_layout.add_instance_variable( name ) #TODO need typing
+
+      if value
+        value = process( value  )
+        raise "value #{value}" #tbc
+      end
+
+      Virtual::Return.new( index )
+    end
+  end
+end