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 at this time.  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.)