make-vector documentation is wrong

[Daniel Colascione]

make-vector claims to "[r]eturn a newly created vector of length 
LENGTH", but we always return a singleton when asked to allocate a 
vector of length zero --- that is, (eq (make-vector 0 nil) (make-vector 
0 nil)). Is this behavior necessary? Is it contractual? We should fix 
either the docstring or the behavior of make-vector, and the latter 
seems better to me because it reduces the number of special cases in the 
system.