Replace coroutine_handle with continuation in executor interface

The executor concept's dispatch() and post() now accept
continuation& instead of std::coroutine_handle<>. A continuation
wraps a coroutine handle with an intrusive next_ pointer, enabling
executors to queue work without per-post heap allocation.

The thread pool's post() previously allocated a work node on every
call (new work(h)). It now links the caller's continuation directly
into an intrusive queue — zero allocation on the hot path.

The continuation lives in the I/O awaitable for caller-handle
posting (delay, async_mutex, async_event, stream), and in
combinator/trampoline state for parent-dispatch and child-launch
patterns (when_all, when_any, timeout, run, run_async). The
IoAwaitable concept and io_awaitable_promise_base are unchanged.

Breaking change: all Executor implementations must update dispatch()
and post() signatures from coroutine_handle<> to continuation&.

Author: mvandeberg

bench/bench.cpp

@@ -81,14 +81,14 @@ class bench_io_context::executor_type
     {
     }
 
-    std::coroutine_handle<> dispatch(std::coroutine_handle<> h) const
+    std::coroutine_handle<> dispatch(continuation& c) const
     {
-        return h;
+        return c.h;
     }
 
-    void post(std::coroutine_handle<> h) const
+    void post(continuation& c) const
     {
-        h.resume();
+        c.h.resume();
     }
 
     void defer(std::coroutine_handle<> h) const