revert to symbols

Parfait::Words were nice, but endless problems with the fact that when
you write “String” you get a string.
Symbols take care of uniqueness at the same time

lib/parfait/space.rb

@@ -56,14 +56,14 @@ module Parfait
     end
 
     def get_main
-      kernel = get_class_by_name "Object"
-      kernel.get_instance_method "main"
+      kernel = get_class_by_name :Object
+      kernel.get_instance_method :main
     end
 
     # this is the way to instantiate classes (not Parfait::Class.new)
     # so we get and keep exactly one per name
     def get_class_by_name name
-      raise "uups #{name}.#{name.class}" unless name.is_a?(Word) or name.is_a?(String)
+      raise "uups #{name}.#{name.class}" unless name.is_a?(Symbol)
       c = @classes[name]
       puts "MISS, no class #{name} #{name.class}" unless c # " #{@classes}"
       c