Write a Vec clone for DSTs

[MaulingMonkey]

Prior art:

• wio::VariableSizedBox (git)
• slice-dst
• dst

I think I like this API shape: AHeaderVec<Header, [Body], Allocator> ?
It might be worth making this do double-duty as the implementation of thin-vecs (putting size/capacity in the preamble)?
AThinVec<Body, Allocator> could be implemented in terms of AHeaderVec<(), [Body], Allocator>?