P3552R3 Add a Coroutine Task Type

Editorial notes:

- the new subclauses are appended at the end of [exec.coro.util],
  not at the end of [exec] as the paper says

- inconsistent default template arguments for the ctor
  in the class declaration (allocator<byte> and the later description
  (allocator<void>); the wording consistently uses allocator<void>

- added "of the" to a sentence about the type of a completion datum:
  "... defines the type *of the* value completion datum"

- the use of the template parameter of the nested type state is Rcvr
  which has two problems:
    1. the ctor also uses the name Rcvr for its template parameter
    2. the class stores an object of type R which is the receiver
       named rcvr
  the most logic fix is to use Rcvr consistently for the state and
  change the name for the ctor's parameter:
    1. change the ctor template parameter type to use R
    2. change the member to use Rcvr
    3. change the type R used for the own-env-t to be
       "...get_env(declval<Rcvr>())..."

Author: dietmarkuehl

source/exceptions.tex

@@ -1141,7 +1141,11 @@
 \item%
 when \tcode{std::execution::get_parallel_scheduler} is called and
 \tcode{std::execution::system_context_replace\-ability::query_parallel_scheduler_backend()}
-returns a null pointer value\iref{exec.par.scheduler}.
+returns a null pointer value\iref{exec.par.scheduler}, or
+
+\item%
+when an exception is thrown from a coroutine \tcode{std::execution::task}\iref{exec.task}
+which doesn't support a \tcode{std::execution::set_error_t(std::exception_ptr)} completion.
 \end{itemize}
 \end{note}