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

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[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.