Add find_mut_equiv function to std::collections::hashmap::HashMap

[Bastacyclop]

I needed this function, so I decided to make it and share it.

I hope it helps.

[alexcrichton]

For previous discussion, see #13205, #11074, #11691, #12135, #14968, and #15220.

I understand this is requested often, but I'm not yet convinced that "add equiv copies of all methods" is the right approach to take here.

[alexcrichton]

Closing due to inactivity, but this is definitely a very real problem that needs a solution!