Refactor of when_all() and when_all_ready().

- Now supports when_all()/when_all_ready() on a std::vector of
  arbitrary awaitables.
- Changed when_all() to use an fmap() composition with result
  of when_all_ready() awaitable to extract the results from the
  when_all_task objects.
  This has the side-effect of meaning that
  'co_await lvalueWhenAllAwaitable' now returns a prvalue result
  rather than an lvalue-reference to the result.
  However, the prvalue tuple/vector will contain a copy of the
  results if awaiting an lvalue (allowing you to await it again
  and get another copy) whereas if you co_await an rvalue awaitable
  then the results will be moved into the tuple/vector.
- Re-added support for passing std::reference_wrapper-wrapped
  awaitables to when_all().
- Renamed when_all_awaitable_counter to when_all_counter.
- Removed when_all_awaitable/when_all_awaitable2 as they are no
  longer needed. Everything is built on when_all_ready_awaitable.

include/cppcoro/detail/when_all_counter.hpp

@@ -13,11 +13,11 @@ namespace cppcoro
 {
 	namespace detail
 	{
-		class when_all_awaitable_counter
+		class when_all_counter
 		{
 		public:
 
-			when_all_awaitable_counter(std::size_t count) noexcept
+			when_all_counter(std::size_t count) noexcept
 				: m_count(count + 1)
 				, m_awaitingCoroutine(nullptr)
 			{}