Users can now determine whether a thread has been started or not. A
started thread might also have already terminated.
Implementation note: We *could* detect this with pthread APIs maybe, but
this is much simpler.
While null StringViews are just as bad, these prevent the removal of
StringView(char const*) as that constructor accepts a nullptr.
No functional changes.
Sometimes you don't care about `joining()` the result of a thread. The
underlying pthread implementation already existed for detaching and
now we expose it to the higher level API.
