Assertions to test that protos are equivalent in MathOpt's sense.
Empty sub-messages (recursively) have no effect on equivalence, except for the
Duration message, otherwise the same as proto equality. Oneof messages have
their fields recursively cleared, but the oneof itself is not, to preserve the
selection. This is similar to C++ EquivToProto, but this function cares about:
* field presence for optional scalar fields,
* field presence for Duration messages (e.g. Duration unset of time limit
means +inf),
* field presence for one_ofs,
and C++ EquivToProto does not.
See normalize.py for details.