Vector refactor, part 3: stack

Hi,
  the attached patch is the missing piece of Vector refactoring: it
replaces users of Stack with std::stack (or std::vector, in one case
where it's needed due to also needing to iterate over the elements),
and removes Vector, Stack and related unit tests.

Farm-build-tested and run-tested.
Code at lp:~squid/squid/stack-refactor.

I've had a strange assert (at HttpHeader.cc:1551, in
~HttpHeaderEntry), but I could not reproduce that nor find anything in
p2 or p3 of the refactor patch which may trigger it.
I'm running more tests at full debugging.

-- 
    Kinkie