Moving sp_thread_pause, sp_thread_sleep to Core

From: Christian Mazakas (christian.mazakas_at_[hidden])
Date: 2023-05-31 18:42:57

Unordered's concurrent flat map contains a hand-rolled
spinlock (graciously provided by Peter Dimov) but it contains
a dependency on SmartPtr for the following two headers:

#include <boost/smart_ptr/detail/sp_thread_pause.hpp>
#include <boost/smart_ptr/detail/sp_thread_sleep.hpp>

I advocate that we move these to Core. They're small, self-contained
and are useful in broader contexts.

Anyone have thoughts or objections?

- Christian