Make ZST checks in core/alloc more readable There's a bunch of these checks because of special handing for ZSTs in various unsafe implementations of stuff. This lets them be `T::IS_ZST` instead of `mem::size_of::<T>() == 0` every time, making them both more readable and more terse. *Not* proposed for stabilization. Would be `pub(crate)` except `alloc` wants to use it too. (And while it doesn't matter now, if we ever get something like #85836 making it a const can help codegen be simpler.)