[Feature request] boost::unordered_flat_map variant with key and value stored separately

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` instead of `std::pair&`.

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

Thanks!