VSIPL16 — Vector destructor should call decrement_count

The ~Vector() destructor function is described as "If this object is the
only one using its block, the block is deleted. Otherwise, its block's
use count is decremented by one."

This is a redundant description of the decrement_block functionality,
and further spreads the description of the block reference-counting
throughout the standard.

It would be much better to either (a) explicitly state that "This calls
the block's decrement_count accessor, which may deallocate the block."

Or, if we do not want to constrain the implementation to literally call
the block method, we can alternately add a paragraph to the block
requirements section describing increment-count and decrement-count
functionality in general (with the increment_count and decrement_count
functions being interfaces to it), and the ~Vector description should
refer to that.