Better map lookups (P3091) using optional<T&>

[JeffGarland]

Here's the issue tracker for (might be private during WG21 meetings) for this paper. It's already in the last phase of the committee for c++29. Looking at the implementation it looks more like a proof of concept then a fully implemented map with the new API's. Note that this proposal also relies on optional ref so is perfect synergy with Beman. The only issue is we'd need to fully implement another standard library component -- but I think we should consider it anyway.

cplusplus/papers#1759

[ssam18]

@JeffGarland - I had a go at implementing this and put together an initial version here: https://github.com/SamareshSingh/map

It covers beman::map::map, beman::map::unordered_map, and beman::map::flat_map with the .get() API from P3091R5. All four overloads are there (key_type and heterogeneous, const and non-const). It pulls in bemanproject/optional as a dependency for the optional<T&> return type.

Tests are passing on GCC 13 with C++23. The flat_map tests are guarded behind a CMake feature check since <flat_map> is only available from GCC 14 onwards.

Happy to move this under the bemanproject org if that makes sense. Would also appreciate any feedback on the design or anything that looks off compared to the paper wording. This is a first pass so there is likely room to improve.