Hello!

It would be great if we could have another variant of boost::unordered_flat_map where the keys and values are stored in separate arrays - "split" storage.
The main motivation for this is to minimise memory wastage due to padding.

eg: key=uint64_t, value=float would mean 4 bytes of padding per key-value pair.

Obviously this will be a further divergence from std::unordered_map's interface.
The main difference being that iterators will return std::pair<const Key&, Value&> instead of std::pair<const Key, Value>&.

However, we believe this will provide a tonne of value to fans of boost::unordered_flat_map.

Thanks!