This type is a hack around the fact that Cell and AtomicUN all have const fn new(val: Inner) -> Self; constructors, but the numberic fundamentals do not. As such, the standard library does not provide a unified construction syntax to turn an integer fundamental into the final type.
This provides a const fn BitElement::<_>::new(R) -> Self; function, implemented only for the BitStore implementors that the crate provides, that the constructor macros can use to turn integers into final values without using mem::transmute. While transmute is acceptable in this case (the types are all #[repr(transparent)]), it is still better avoided where possible.
As this is a macro assistant, it is publicly exposed, but is not public API. It has no purpose outside of the crate’s macros.