Change context-mode's default to new mode 4.

This new mode uses a copy of the TOPLEVEL_BINDING. This is compatible with refinements (contrary to mode 3), while keeping nested IRB sessions separate

lib/irb/init.rb

@@ -108,7 +108,7 @@ def IRB.init_config(ap_path)
     @CONF[:PROMPT_MODE] = (STDIN.tty? ? :DEFAULT : :NULL)
     @CONF[:AUTO_INDENT] = true
 
-    @CONF[:CONTEXT_MODE] = 3 # use binding in function on TOPLEVEL_BINDING
+    @CONF[:CONTEXT_MODE] = 4 # use a copy of TOPLEVEL_BINDING
     @CONF[:SINGLE_IRB] = false
 
     @CONF[:LC_MESSAGES] = Locale.new

lib/irb/workspace.rb

@@ -51,11 +51,13 @@ def initialize(*main)
           end
           @binding = BINDING_QUEUE.pop
 
-        when 3	# binding in function on TOPLEVEL_BINDING(default)
+        when 3	# binding in function on TOPLEVEL_BINDING
           @binding = eval("self.class.send(:remove_method, :irb_binding) if defined?(irb_binding); private; def irb_binding; binding; end; irb_binding",
                           TOPLEVEL_BINDING,
                           __FILE__,
                           __LINE__ - 3)
+        when 4  # binding is a copy of TOPLEVEL_BINDING (default)
+          @binding = TOPLEVEL_BINDING.dup
         end
       end