Summary:
This diff extracts the key destructuring logic in F14's emplace
functions into standalone helpers that can be used by other container
types (like sorted_vector_{set,map}).  It also adds better PMR compliance, by
using a stateful allocator to construct a temporary if that is required
to be able to perform the search.

This diff also cleans up some of the testing code in folly/container/test
that had issues if included in multiple places.

Reviewed By: yfeldblum

Differential Revision: D16655131

fbshipit-source-id: 5a6f57ac346d1fd5928e0f737110760b0c518974