std::unordered_map insert with hint

It is an interface compatibility issue. Basically, the design is done considering the interface of std::map.

In other words, for std::unordered_map it does not differ a hint is provided or not.

Additional Information from the comments here:

The interface compatibility is very important because being able to quickly/easily switch between map and unordered_map provides the valuable flexibility of painlessly transition since performance is often the deciding factor in choosing one over the other.

The hint allows the unordered map implementation to do a value compare first to see if the hint works. This avoids having to do the hash function which can be more costly than a compare operation.