CP.25 raii_thread

The PR for this is now ready to merge into GSL here: microsoft/GSL#504
IMO the design looks fine. But do we really want to use the name "raii_thread"? Yes, that's technically what it is (for those who like using that acronym), but why not "joining_thread" or "scoped_thread"? ... or, best of all, "thread"!

Proposed resolution: Rename "raii_thread" to just gsl::thread. This type is what std::thread ought to have been, and it's identical to std::thread except that gsl::thread has a properly-joining destructor and does not have detach. And this is what namespaces are for. :)

CP.26 detached_thread

The GSL PR has split out "raii_thread" because I suggested "detached_thread" was problematic and less recommendable. So we should also do something about CP.26 which recommends "detached_thread" -- I think detaching is an antipattern we should not encourage, as Hans and others have pointed out many times (e.g., orderly shutdown is at best problematic and usually impossible).

Proposed resolution:

• Drop "gsl::detached_thread" and the current CP.26.
• Replace CP.26 with the inverse Guideline that says "never detach threads" (that’s an unsafe antipattern). Include an example showing how you can never safely join or interact with it during static destruction. Enforcement: Flag any call to std::thread::detach. What to do instead: Just stick a normal gsl::thread in a global object or container and then ignore it if you want… but it means better more-orderly shutdown by default plus the ability to actually join with it if you do want to refer to it later.