firebase-ios-sdk

Continue work on ReferenceSet (#2213)

* Migrate FSTDocumentReference to C++

* Change SortedSet template parameter ordering

Makes it easier to specify a comparator without specifying what the
empty member of the underlying map is.

* Migrate MemoryMutationQueue to C++ references by key

* migrate.py

* CMake

* Finish porting ReferenceSet

* Swap reference set implementation

* Port MemoryQueryCache to use ported ReferenceSet

* Port FSTReferenceSetTest

* Port usage for limbo document refs

* Port LRU and LocalStore usages

* Remove FSTReferenceSet and FSTDocumentReference

* Style

* Add newline