PR 90904 notes that auto_vec is unsafe to copy and assign because
the class manages its own memory but doesn't define (or delete)
either special function.  Since I first ran into the problem,
auto_vec has grown a move ctor and move assignment from
a dynamically-allocated vec but still no copy ctor or copy
assignment operator.

The attached patch adds the two special functions to auto_vec along
with a few simple tests.  It makes auto_vec safe to use in containers
that expect copyable and assignable element types and passes bootstrap
and regression testing on x86_64-linux.

Martin