Splitting NameExpression into three, Known,Local,Argument

The decision which to use can be made higher up, in ruby, and so it
should.

lib/register/builtin/compile_helper.rb

@@ -4,7 +4,7 @@ module Register
     module CompileHelper
 
       def self_and_int_arg(compiler , source)
-        me = compiler.process( Vm::Tree::NameExpression.new( :self) )
+        me = compiler.process( Vm::Tree::KnownName.new( :self) )
         int_arg = load_int_arg_at(compiler , source , 1 )
         return me , int_arg
       end