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/risc/test_builder.rb

@@ -44,14 +44,14 @@ module Risc
     def test_returns_slot
       r2 = RegisterValue.new(:r2 , :Message)
       r2.builder = @builder
-      built = @builder.build{ r2 << space[:first_message] }
+      built = @builder.build{ r2 << space[:next_message] }
       assert_equal SlotToReg , built.class
       assert_equal :r1 , built.array.symbol
     end
     def test_returns_slot_reverse
       r2 = RegisterValue.new(:r2 , :Message)
       r2.builder = @builder
-      built = @builder.build{ r2 << space[:first_message] }
+      built = @builder.build{ r2 << space[:next_message] }
       assert_equal SlotToReg , built.class
       assert_equal :r1 , built.array.symbol
     end