Summary:
`hash_range(c.begin(), c.end()` combines hashes of individual elements of a
container in an ordered manner. This diff provides the equivalent for
unordered containers.

Unlike `hash_range`, `commutative_hash_combine_range` defaults to `folly::Hash`
as its hasher; it mixes the individual hash if the `hasher` is not deemed
avalanching.

It uses a commutative accumulator described in this paper:
https://www.preprints.org/manuscript/201710.0192/v1/download
In the experiments in the paper, the symmetric polynomial yielded a better
spread of hash values and lower collision rates than `+` or `xor`.

Reviewed By: yfeldblum

Differential Revision: D9688687

fbshipit-source-id: c812b25975a53a868d98f78645146cb8bdbb5c32