set up ivar_assignment correctly

for statement tests
and write mom tests
also implement to_mom
and fix local (self) bug

test/vool/to_mom/test_ivar.rb

@@ -0,0 +1,34 @@
+require_relative "helper"
+
+module Vool
+  class TestIvarMom < MiniTest::Test
+    include MomCompile
+
+    def setup
+      Risc.machine.boot
+      @method = compile_first_method( "@a = 5")
+    end
+
+    def test_class_compiles
+      assert_equal Mom::SlotConstant , @method.first.class , @method
+    end
+    def test_slot_is_set
+      assert @method.first.left
+    end
+    def test_slot_starts_at_message
+      assert_equal :message , @method.first.left[0]
+    end
+    def test_slot_gets_self
+      assert_equal :self , @method.first.left[1]
+    end
+    def test_slot_assigns_to_local
+      assert_equal :a , @method.first.left[-1]
+    end
+    def test_slot_assigns_something
+      assert @method.first.right
+    end
+    def test_slot_assigns_int
+      assert_equal IntegerStatement ,  @method.first.right.class
+    end
+  end
+end