Rename SlotDefinition to Slot

And the derived XXDefinitions to XXSlot

Just to be more consistent
And possibly free the Definition for the Language side

test/sol/lambdas/test_assign.rb

@@ -73,7 +73,7 @@ module SolBlocks
     def test_assigns_move
       @ins = compile_main_block( "@a = arg")
       assert_equal SlotMachine::SlotLoad , @ins.class , @ins
-      assert_equal SlotMachine::MessageDefinition , @ins.right.class , @ins
+      assert_equal SlotMachine::MessageSlot , @ins.right.class , @ins
     end
   end
 

test/sol/lambdas/test_if_condition.rb

@@ -12,7 +12,7 @@ module SolBlocks
       assert_equal TruthCheck , @ins.next(3).class
     end
     def test_condition_is_slot
-      assert_equal MessageDefinition , @ins.next(3).condition.class , @ins
+      assert_equal MessageSlot , @ins.next(3).condition.class , @ins
     end
     def test_simple_call
       assert_equal SimpleCall , @ins.next(2).class

test/sol/lambdas/test_while_simple.rb

@@ -15,7 +15,7 @@ module SolBlocks
       assert_equal TruthCheck , @ins.next.class , @ins
     end
     def test_condition_is_slot
-      assert_equal MessageDefinition , @ins.next.condition.class , @ins
+      assert_equal MessageSlot , @ins.next.condition.class , @ins
     end
     def test_array
       check_array [Label, TruthCheck, SlotLoad, Jump, Label,