* add a new property `timestampsInSnapshotsEnabled` to `FirestoreSettings`, `false` by default;
* add a verbose warning message urging users to opt into the new behavior;
* set `timestampsInSnapshotsEnabled` to true in the integration tests to reduce the verbose console
spam during the test run and make sure the flag won't break anything once it's flipped.
* Move all Firestore files to Objective-C++
* Update project file references
* Don't use module imports from Objective-C++
* Use extern "C" for C-accessible globals
* Work around more stringent type checking in Objective-C++
* NSMutableDictionary ivars aren't implicitly casted to NSDictionary
* FSTMaybeDocument callback can't be passed a function that accepts
FSTDocument
* NSComparisonResult can't be multiplied by -1 without casting
* Add a #include <inttypes.h> where needed
* Avoid using C++ keywords as variables
* Remove #if __cplusplus guards
Michael Lehenbauer