firebase-ios-sdk

Use three-way comparisons in C++ (#2884)

* Remove last traces of Objective-C from document_key.h

* Generalize empty base optimization into CompressedMember

Remove ComparatorHolder.

* Add Comparator<T>.Compare

This makes C++ more like the other platforms, where three-way comparison
is the default.

For now, don't remove the less-than definition of operator()--this
allows these comparators to be used in both circumstances.

* Remove KeyComparator

With the transition between less-than and three-way comparison adapting
this class isn't worth it.

While technically this is losing use of std::lower_bound, the
performance cost is negligible because these arrays are guaranteed to be
always small. This also makes ArraySortedMap work more like it does in
the other ports.

* Add Compare to the remaining Comparators

* Migrate remaining comparator usage in immuntable to three-way

* Use absl::bit_cast in comparison

* Make Comparable depend upon CompareTo.

* Use Comparable in more places

* Remove Comparator::operator()

* Move things around in comparison.h

Move:
  * Objective-C ComparisonResult functions nearer to others
  * DefaultComparator to above the first specializations

* Make Comparison on by default, delegating to CompareTo if available

* Move NSString Comparator into comparison.h

* Remove no longer required Comparator classes

* Make FieldValue support Comparable