Merge `nzst::*` and `zsty::*` into `fat::*`?

[MaulingMonkey]

Motivating example:

let u32 = ABox::new_in(42_u32, Malloc);
let zst = ABox::new_in((),     Malloc);

It would be nice if the former compiled while the latter didn't. This cannot be particularly well accomplished by separating nzst::* from zsty::*. For another example, these should both compile:

let u32 = ABox::new_in(42_u32, DangleZst(Malloc));
let zst = ABox::new_in((),     DangleZst(Malloc));

TODO

• [x] Widen thin to accept usize
• [x] Reimpl everything in terms of zsty
• [x] Drop nzst
• [x] Rename nzst -> fat
• [x] ABox ZST support assertions

[MaulingMonkey]

c781389 ABox: ZST checks