This is part #1 of a two patch series to does two things:

  1) get rid of the crock that was struct continuation_arg, by replacing
     it by a void*, and a structure per continuation.

  2) Make continuations have a mechanism to free the heap allocated
     arguments.

I don't think this patch needs much explanation.

Has a nice code size drop, and legibility increase, I think.

Tested on x86_64-unknown-linux-gnu {sync,async}

OK?

-- 
Pedro Alves