Add utility function to create frozenset with unique values

When used instead of a plain call to “frozenset”, this would have
avoided the issue fixed in commit e2dd6ece. The new function is located
in the “compat” module as it will be used at module load time in most
places and should therefore reside in a place with very few
dependencies.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>