Cache Gem::Version instances for each version string

This prevents having numerous instances of the same value object. In
particular, the use of String#scan in Version#segments is a hot spot. We
could perform the caching there but it seems reasonable to cache the
entire object.

lib/rubygems/version.rb

@@ -153,7 +153,10 @@ class Gem::Version
   ##
   # A string representation of this Version.
 
-  attr_reader :version
+  def version
+    @version.dup
+  end
+
   alias to_s version
 
   ##
@@ -183,6 +186,12 @@ def self.create input
     end
   end
 
+  @@all = {}
+
+  def self.new version
+    @@all[version] ||= super
+  end
+
   ##
   # Constructs a Version from the +version+ string.  A version string is a
   # series of digits or ASCII letters separated by dots.