Replace Kernel#pp after PP class is defined.

Avoid a race condition which a context switch
occur after replacing Kernel#pp but before
defining PP class.

Following patch, inserting sleep, makes
this problem reproducible.

```
Index: lib/pp.rb
===================================================================
--- lib/pp.rb	(revision 60960)
+++ lib/pp.rb	(working copy)
@@ -26,6 +26,7 @@ module Kernel
   end
   undef __pp_backup__ if method_defined?(:__pp_backup__)
   module_function :pp
+  sleep 1 # thread context switch
 end
 
 ##
```

With the above patch, "uninitialized constant Kernel::PP" can
happen as as follows.

```
% ./ruby -w -Ilib -e '
t1 = Thread.new {
  Thread.current.report_on_exception = true
  pp :foo1
}
t2 = Thread.new {
  Thread.current.report_on_exception = true
  sleep 0.5
  pp :foo2
}
t1.join rescue nil
t2.join rescue nil
'
#<Thread:0x000055dbf926eaa0@-e:6 run> terminated with exception:
Traceback (most recent call last):
	3: from -e:9:in `block in <main>'
	2: from /home/ruby/tst2/ruby/lib/pp.rb:22:in `pp'
	1: from /home/ruby/tst2/ruby/lib/pp.rb:22:in `each'
/home/ruby/tst2/ruby/lib/pp.rb:23:in `block in pp': uninitialized constant Kernel::PP (NameError)
:foo1
```



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60961 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/pp.rb

@@ -2,32 +2,6 @@
 
 require 'prettyprint'
 
-module Kernel
-  # Returns a pretty printed object as a string.
-  #
-  # In order to use this method you must first require the PP module:
-  #
-  #   require 'pp'
-  #
-  # See the PP module for more information.
-  def pretty_inspect
-    PP.pp(self, ''.dup)
-  end
-
-  # prints arguments in pretty form.
-  #
-  # pp returns argument(s).
-  alias __pp_backup__ pp if method_defined?(:pp)
-  def pp(*objs)
-    objs.each {|obj|
-      PP.pp(obj)
-    }
-    objs.size <= 1 ? objs.first : objs
-  end
-  undef __pp_backup__ if method_defined?(:__pp_backup__)
-  module_function :pp
-end
-
 ##
 # A pretty-printer for Ruby objects.
 #
@@ -562,3 +536,30 @@ end
     end
   }
 }
+
+module Kernel
+  # Returns a pretty printed object as a string.
+  #
+  # In order to use this method you must first require the PP module:
+  #
+  #   require 'pp'
+  #
+  # See the PP module for more information.
+  def pretty_inspect
+    PP.pp(self, ''.dup)
+  end
+
+  # prints arguments in pretty form.
+  #
+  # pp returns argument(s).
+  alias __pp_backup__ pp if method_defined?(:pp)
+  def pp(*objs)
+    objs.each {|obj|
+      PP.pp(obj)
+    }
+    objs.size <= 1 ? objs.first : objs
+  end
+  undef __pp_backup__ if method_defined?(:__pp_backup__)
+  module_function :pp
+end
+