Emplace with precalculated hash might be helpful

[lvnohanyan]

If we have precalculated hash, emplace/insert might be helpful as well?

[lucasexe]

I'd also like to have this enhancement.

Context:
I'm working in a multi-thread environment and I need to unlock the mutex as soon as possible.
Having this override for emplace/insert it would be possible to compute the hash before locking the mutex.