Merged pdd25cx branch

[Allison Randal]

I just merged in the pdd25cx branch. Resolving all the conflicts took 
several hours (124 files, 1 minute or more per file...), but otherwise 
it went smoothly.

The biggest changes you'll notice are the new exception system, a vastly 
reduced usage of the remaining stack (though it's not completely removed 
yet), and the fact that pushaction subs no longer fire on scope exit 
(pushaction will likely be deprecated).

Keep an eye out for exception-related problems in the next couple of 
weeks before the release.

Allison