swap was using an incorrect condition to determine when to apply an optimization
(not swapping the full direct[] when swapping two indirect prevectors).
Rather than correct the optimization I'm removing it for simplicity. Removing
this optimization minutely improves performance in the typical (currently only)
usage of member swap(), which is swapping with a freshly value-initialized
object.
Fixes a bug in which pop_back did not call the deleted item's destructor.
Using the most general erase() implementation to implement all the others
prevents similar bugs because the coupling between deallocation and destructor
invocation only needs to be maintained in one place.
Also reduces duplication of complex memmove logic.
