Move the Main instruction from register to virtual

also needs a branch in register.
This way the register level is self sufficient
(noticed while writing debugger)

lib/register/instructions/branch.rb

@@ -0,0 +1,12 @@
+module Register
+
+
+  # a branch must branch to a block.
+  class Branch < Instruction
+    def initialize to
+      raise "No block" unless to
+      @block = to
+    end
+    attr_reader :block
+  end
+end

lib/register/instructions/register_main.rb

@@ -1,19 +0,0 @@
-module Register
-
-  # This starts the register machine machine at the given function.
-
-  # The implementation is most likely a jump/branch , but since we have the extra layer
-  # we make good use of it, ie give things descriptive names (what they do, not how)
-
-  class RegisterMain < Instruction
-    include Positioned
-
-    def initialize method
-      @method = method
-    end
-    attr_reader :method
-    def word_length
-      4
-    end
-  end
-end