[boost] Another Phoenix review

I vote yes. I learned functional programming from spirit and have used phoenix for few years, even though it was tucked away in the library. I'm happy that Joel moved it as a first level citizen of boost. My favorite thing about phoenix, or functional programming in general, is that it encourages you to think out of the box. I actively seek out ways to make my code more "functional', and phoenix is an important part of my toolbox. Phoenix and spirit are in my opinion two of the top libraries on boost. Phoenix is mature and deserves to be at the root of the boost tree. Thanks to the authors. Tom Brinkman