Summary:
This diff adds heterogeneous lookup handling for `ConcurrentHashMap` (`find()`
and `at()`), which allows lookups on keys that the hasher and key equal
functors accept but are not exactly the same as the `KeyType` of the map.
Common usage of the feature include looking up with a `std::string_view` key in
a `map<string, X>`, thereby avoiding creating an unnecessary temporary string
object.

Reviewed By: yfeldblum

Differential Revision: D25250409

fbshipit-source-id: 78dad163d623794b179c1691180b54cf109784ee