[PATCH] refactor Vector

Hi,
  the attached patch (merge from lp:~squid/squid/vector-refactor) is
an attempt to refactor Vector and its clients so that:
- clients of Vector don't break layering
- the Vector API more closely matches std::vector

The eventual aim is to replace Vector with std::vector, only if some
more accurate measurements (in the lp:~squid/squid/vector-to-stdvector
branch) show that this wouldn't cause performance degradations.

Don't be fooled by the size of the patch; it is big because there's
lots of context.

Thanks

-- 
    Kinkie