Release 0.82.0

Added

Add a serialized raw array content field to the Array proto.
A function to DPCompositeKeyCombiner that allows retrieval of an ordinal.
Intended for use by the closed-domain DP histogram aggregation core.
Constants for invalid ordinals and default l0_bound_.

How DPCompositeKeyCombiner handles invalid l0_bound_ values.
The default l0_bound_ value in DPCompositeKeyCombiner to new constant.
Organization of DP histogram code. Previously, open-domain histogram class +
factory class lived side-by-side in dp_group_by_aggregator.h/cc. Now split
into dp_open_domain_histogram.h/cc and dp_group_by_factory.h/cc, which
will ease future addition of code for closed-domain histogram.
Moved tff.federated_secure_modular_sum to the mapreduce backend, use
tff.backends.mapreduce.federated_secure_modular_sum instead.