Feature request: support heterogenous lookup for maps

[chenlijun99]

Hi,
I think heterogeneous lookup could be a nice addition. Main use case I think is when working with etl::unordered_map and etl::unordered_set where we wouldn't need to allocate the key to find items.

[jwellbelove]

Heterogenous lookup has been implemented for all of the map/set containers, except for etl::unordered_map/set & etl::unordered_multimap/multiset.