Implement P3149R11 and P3815R1

[ispeters]

This PR will implement P3149R11 patched with P3815R1.

• [exec.scope.concepts]
  • concept scope_association
  • concept scope_token
• [exec.associate]
  • associate algorithm
• [exec.stop.when]
  • stop-when algorithm
• [exec.spawn]
  • spawn algorithm
• [exec.spawn.future]
  • spawn_future algorithm
• [exec.counting.scopes]
  • simple_counting_scope
  • counting_scope

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[ericniebler]

/ok to test fd9d04b

[ispeters]

The latest push includes an implementation of stdexec::associate with some tests, but the tests are incomplete so I haven't marked associate as done in the summary.

I have also tried to address the build failures on the previous iteration, but I don't have a local test environment for the failing build types so I haven't tested the fixes locally, other than to confirm that they still build with my local Clang (i.e. Homebrew clang version 21.1.7).

[ispeters]

Rebased on #1717.

[ericniebler]

/ok to test f786c00

[ericniebler]

/ok to test 97d79ee

[ispeters]

I managed to get GCC 12 installed on my Mac and it repro'd the ICE and the build failure in test_stop_when.cpp; I think I've addressed the build failures in GCC, nvcc, and MSVC.

[ispeters]

simple_counting_scope still needs more tests, but I did discover that [exec.simple.counting.mem], paragraph 9 has a bug: we need to move the scope to the joined state and return true if count is zero, regardless of the value of state.

[ericniebler]

/ok to test cc867f4

[ispeters]

I think I've fixed the build breaks in old gcc, nvc++, and MSVC. Getting a repro of the gcc 11 builds was quite a pain—I can't figure out how to get gcc 11 to run properly on macOS Tahoe so I had to use container, which I learned about here. Fixing MSVC is a matter of guessing. I can't run nvc++ on my Mac, but it turns out it's available on Godbolt, and this was very helpful.

I've updated __counting_scopes.hpp rather extensively; I renamed some of the variables in simple_counting_scope to make the naming more consistent, and I've thoroughly documented both the paragraphs in the standard being implemented and my justifications for weaker-than-seq_cst memory orderings.

[ispeters]

Pull the guts of simple_counting_scope into a __base_scope and implement both simple_counting_scope and counting_scope in terms of it.

[ispeters]

Do some template hoisting and renaming, and replace a single virtual function with a function pointer to cut down on RTTI.