MAINT-5357: Introduce and populate llcoro:: namespace.

To date, the coroutine helper functions in lleventcoro.h have been in the
global namespace. Migrate them into llcoro namespace, and fix references.
Specifically, LLVoidListener => llcoro::VoidListener, and voidlistener(),
postAndWait(), both waitForEventOn(), postAndWait2(), errorException() and
errorLog() have been moved into llcoro.
Also migrate new LLCoros::get_self() and Suspending to llcoro:: namespace.
While at it, I realized that -- having converted several lleventcoro.h
functions from templates (for arbitrary 'self' parameter type) to ordinary
functions, having moved them from lleventcoro.h to lleventcoro.cpp, we can now
migrate their helpers from lleventcoro.h to lleventcoro.cpp as well. This
eliminates the need for the LLEventDetail namespace; the relevant helpers are
now in an anonymous namespace in the .cpp file: listenerNameForCoro(),
storeToLLSDPath(), WaitForEventOnHelper and wfeoh().