rename register to risc

seems to fit the layer much better as we really have a very reduced
instruction set

test/parfait/test_attributes.rb

@@ -3,7 +3,7 @@ require_relative "../helper"
 class TestAttributes < MiniTest::Test
 
   def setup
-    Register.machine.boot
+    Risc.machine.boot
     @space = Parfait.object_space
     @mess = @space.first_message
     @type = @mess.get_type

test/parfait/test_class.rb

@@ -3,7 +3,7 @@ require_relative "../helper"
 class TestClass < MiniTest::Test
 
   def setup
-    Register.machine.boot
+    Risc.machine.boot
     @space = Parfait.object_space
     @try = @space.create_class :Try , :Object
   end

test/parfait/test_message.rb

@@ -3,7 +3,7 @@ require_relative "../helper"
 class TestMessage < MiniTest::Test
 
   def setup
-    Register.machine.boot
+    Risc.machine.boot
     @space = Parfait.object_space
     @mess = @space.first_message
   end

test/parfait/test_named_list.rb

@@ -3,7 +3,7 @@ require_relative "../helper"
 class TestNamedLists < MiniTest::Test
 
   def setup
-    Register.machine.boot
+    Risc.machine.boot
     @space = Parfait.object_space
     @named_list = @space.first_message.locals
     @type = @named_list.get_type

test/parfait/test_space.rb

@@ -3,7 +3,7 @@ require_relative "../helper"
 class TestSpace < MiniTest::Test
 
   def setup
-    @machine = Register.machine.boot
+    @machine = Risc.machine.boot
     @space = Parfait.object_space
   end
 

test/parfait/test_word.rb

@@ -3,7 +3,7 @@ module Parfait
 class TestEmptyWord < MiniTest::Test
 
   def setup
-    Register.machine.boot
+    Risc.machine.boot
     @word = Parfait::Word.new(0)
   end
   def test_word_create
@@ -29,7 +29,7 @@ end
 class TestWord < MiniTest::Test
 
   def setup
-    Register.machine.boot
+    Risc.machine.boot
     @word = Parfait::Word.new(5)
   end
   def test_len