[FixedMap/EnumMap] Fix emplace to work with non-copyable/non-moveable types and piecewise construction

[alexkaratarakis]

No description provided.

[younesr1]

im curious, how does std::pair<K, V> as_pair{std::forward<Args>(args)...}; work if the parameter pack has sizeof...(Args) != 2 values?

[alexkaratarakis]

This raises a good point. Since std::map stores std::pair, the emplace target is a pair.
Hence, the original code was creating a std::pair and then using that, so possible compilation issues would be handled by constructing the std::pair.

This would reject parameter packs that were not compatible (including sizeof...(Args) != 2 although you can actually have different count). However, this is problematic for non-moveable/non-copyable types due to the temporary pair.

I have revised the code and tests to be more explicit about what is supported.