Add vec![ptr::null{,_mut}(); n] optimization, like vec![0; n]

vec![0; n], via implementations of SpecFromElem, has an optimization
that uses with_capacity_zeroed instead of with_capacity, which will use
calloc instead of malloc, and avoid an extra memset.

This adds the same optimization for vec![ptr::null(); n] and
vec![ptr::null_mut(); n], assuming their bit value is 0 (which is true
on all currently supported platforms).

This does so by adding an intermediate trait IsZero, which looks very
much like nonzero::Zeroable, but that one is on the way out, and doesn't
apply to pointers anyways.

Adding such a trait allows to avoid repeating the logic using
with_capacity_zeroed or with_capacity, or making the macro more complex
to support generics.

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