This reverts commit 7bc3a9eeae.
I'd hoped that `thread_local!` would force our code to be single-
threaded, but apparently it doesn't. With a global mutable static, it
seems like we have to include some kind of thread-safety to avoid
`unsafe` code, and an atomic provides the kind of safety we actually
want.
In the last revision, the next element serial number was thread-safe,
but that might be overdoing it. I suspect that elements will only ever
be created from the main thread.
