unify space collection attribute naming

currently space is still acting as a sort of memory manager.
For proper linking, all objects must be reachable from space, hence the plural versions like messages and addresses (even they are instances, it is the list that is important)
To dish out instance to use, the head must be kept, ie next_XXX for intergers, return addresses and messages

test/parfait/type/test_basic.rb

@@ -5,7 +5,7 @@ module Parfait
 
     def setup
       super
-      @mess = @space.first_message
+      @mess = @space.next_message
       assert @mess
       @type = @mess.get_type()
     end

test/parfait/type/test_message.rb

@@ -5,7 +5,7 @@ module Parfait
 
     def setup
       super
-      @mess = @space.first_message
+      @mess = @space.next_message
     end
 
     def test_message_type