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