P1222R4 `<flat_set>`

[StephanTLavavej]

P1222R4 <flat_set>
LWG-3751 Missing feature macro for flat_set
LWG-3774 <flat_set> should include <compare>
LWG-3786 Flat maps' deduction guides need to default Allocator to be useful
LWG-3803 flat_meow constructors taking KeyContainer lack KeyCompare parameter
LWG-3879 erase_if for flat_{,multi}set is incorrectly specified
LWG-3884 flat_meow is missing allocator-extended copy/move constructors
LWG-3987 Including <flat_meow> doesn't provide std::begin/end

Feature-test macro:

#define __cpp_lib_flat_set 202207L

Internal VS-PR-420058 has taught the IDE about this new extensionless header - thanks @CaseyCarter!

[Saalvage]

I have just started on a branch implementing this feature. :)
It will need a lot more tests and a few improvements but I'd love to contribute it!

[StephanTLavavej]

We're starting a multi-collaborator feature branch, feature/flat_set, with @Saalvage's implementation, so that it can be refined and test coverage can be added.