New development branch: stree-branch

[Matt Austern]

I have just created stree-branch, which I'm about to document in 
cvs.html#devbranches.  The purpose of this branch is to speed up 
compilation by replacing some front end data structures with a compact 
representation, which can be lazily expanded to the full tree form.

(The premise here is that header files contain many declarations that 
are never used.  They'll never have to be expanded, so we can save time 
and memory.  This is especially useful when combined with PCH.)

I expect to be making the first checkin into this branch today.

Checkin policy for this branch: please get approval from Geoff Keating, 
Robert Bowdidge, or me.

			--Matt