when_all: Conditional Stop Source

[RobertLeahy]

This commit implements P4217R1 and P4269R0 by making the following changes to when_all:

• when_all() is now expression-equivalent to just()
• when_all(s) is now expression-equivalent to auto(s) (i.e. it decay-copies s)
• The operation state for when_all(s0, ..., sN) now only contains a stop source if:
  • Any of s0, ..., sN can complete unsuccessfully in the ambient environment, or
  • Decay-copying the values sent by any of s0, ... sN can throw an exception

The last bullet prevents when_all from "hallucinating" set_stopped: Previously when_all unconditionally managed a stop source, and injected stop tokens therefrom into its children. Consider when_all(s0, s1) where s0 and s1:

• Have only set_value_t() as a completion signature in the ambient environment, and
• Have set_value_t() and set_stopped_t() as completion signatures in an environment with an inplace_stop_token

when_all(s0, s1) plainly does not need a stop source because neither s0 nor s1 will ever complete unsuccessfully (see bullet one above). However when_all(s0, s1) would previously create a stop source anyway, and inject tokens therefrom into s0 and s1. This would cause s0 and s1 to report an additional stopped completion (see bullet two above) which would cause when_all to report set_value_t() and set_stopped_t() as completion signatures even though the latter is never actually emitted (because s0 and s1 don't fail except by stop request, but the only way for a stop request to be emitted is for either of them to fail).

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