Performance improvements to multibyte utils

In the 2-3-stable branch, the ActiveSupport::Multibyte.clean() was doing this:

stripped = []; for c in string.split(//)
  stripped << c if expression =~ c
end; stripped.join

That was creating tons of garbage. Actually, it was the single biggest creator of garbage in our requests (around 4500-6000 objects).

It was replaced with

string.split(//).grep(expression).join

...in master, which reduces the amount of garbage by about 70%. I backported that change to 2-3-stable:

http://github.com/jamesgolick/rails/commit/ff2f98173d398cc40f5d6e2f...

And I also added a patch of my own to reduce the amount of garbage in ActiveSupport::Multibyte.verify():

http://github.com/jamesgolick/rails/commit/eb96874f97b8e11f37f9c6c7...

and backported that too:

http://github.com/jamesgolick/rails/commit/69a5cad7f7c7d6f7498dd87a...