Using these on const atoms violates const-correctness, but unfortunately
changing the macros would technically break the API.  Ultimately, this code
should be moved outside the (effective) spec so that it can be improved in
general, but for now, just inline the macro bodies and add the necessary const
qualifiers.

Unfortunately the function effect checking added in clang 20 won't infer a
function is nonblocking if it has a static constant variable (because the
compiler might insert a lock to initialize it, or it might have an advanced
constructor in C++). This isn't a big loss here, so just inline the value.

There's no need to separately check the size and type here, since these are in
the header of both atoms and can be compared along with everything else by a
single call to memcmp().