Polish std::generator

[CaseyCarter]

This should complete the generator feature branch and get it ready to merge. There are very few product code changes here, it's mostly expanded test coverage with some cleanup and reorganization. There's also some documentation of the "stack of coroutines" and the memory layout of allocations that will make it easier for folks to get up to speed on how the code works under the covers.

It's broken down nicely by commit for ease of review; I'll replicate the commit messages here as an overview. Comments without specific context refer to P2502R2_generator where most changes were made. The P2502R2_generator_iterator and P2502R2_generator_promise tests were already very nicely complete.

• Update generator template argument mandates
  • Update citations to WG21-N4988
  • Reorder checks to specification order
  • Make _RRef more obviously reflect the wording for RRef in WG21-N4988 [coro.generator.class]/1.4.
• Allocator testing updates
  • Simplify and correct StatelessAlloc
    • Deriving publicly from std::allocator not a great idea, as witnessed by the recent addition of allocate_at_least.
    • deallocate should be noexcept for a Cpp17Allocator.
    • We don't need to reimplement std::allocator when we can simply use it.
    • The domain of equality for Cpp17Allocators is an entire rebind family, i.e., StatelessAlloc<T> and allocator_traits<StatelessAlloc<T>>::rebind_alloc<U> must be comparable.
  • PascalCase stateful_alloc for consistency and move into the header with StatelessAlloc.
• Promise test tweaks
  • "whose member await_ready returns false" in WG21-N4988 [coro.generator.promise]/11 implies the return type is exactly bool, not "convertible to bool". Update test_yield_elements_of_range accordingly.
  • Move non-portable size check from P2502R2_generator into P2502R2_generator_promise
• Expand and complete static_checks: Implement a complete generator traits in the header to use in both P2502R2_generator_iterator and P2502R2_generator's static_checks.
• Create generic test_one template which takes a generator, a description of its static properties, and the expected result of piping the generator through a provided range adaptor. test_one validates the static properties with static_checks, and confirms the output is as expected.
• Extract test cases for weird reference types (mutable lvalue and rvalue references) from main into new function test_weird_reference_types.
• Several small tweaks:
  • It's no longer significant that co_upto wasn't an example in the proposal; strike the comment.
  • Enforce co_upto's precondition so it's nicely documented.
  • Consistently prefer same_as to is_same_v.
  • Regroup calls to test cases in main topically and title each category.
• Reorganize test code to agree with call order in main. [This is the largest individual commit; it is a pure reordering.]
• All product code assertions now depend on _CONTAINER_DEBUG_LEVEL. These are all simple O(1) checks, there's no reason not to promote them from _DEBUG to _CONTAINER_DEBUG_LEVEL.
• Clarify allocation mechanisms, including some fancy memory layout diagrams.
  • Rename _Promise_allocator to _Coro_promise_allocator to avoid any confusion with _Promise. Rename its template parameter _Allocator to _Proto_allocator to avoid confusion with the rebound allocator type _Alloc.
  • Expand static_assert message in operator new. If and when _Coro_promise_allocator is reused by other coroutine types we can worry about making the message more generic.
  • Extract block size computations.
• Element_awaiter must direct-non-list-initialize its stored object, per WG21-N4988 [coro.generator.promise]/7.
• Document the "stack of coroutine handles" in a code comment with another work of art.