Extract useful bits of FSTUserDataConverter for use in C++:
* Migrate FSTParsedSetData to core::ParsedSetData
* Migrate FSTParsedUpdateData to core::ParsedUpdateData
* Migrate FSTParseContext to core::ParseContext
* Split ParseAccumulator out of ParseContext to make ownership easier
Also:
* Remove dead code (handled in parseSentinelFieldValue)
* Flatten App/iOS/Supporting Files into the parent
* remove conversions in `Local` and `Remote`
* replace `FSTSnapshotVersion` in `Model`
* replace `FSTSnapshotVersion` in the rest
* let test check equal with C++ equal
* fix newly added code
* address changes
* port FieldMask to C++
* address changes
* address changes
* fix test
* address change
* Port transform operations (FSTTransformOperation, FSTServerTimestampTransform) to C++
* address changes
* address changes
* address changes
* implement `FieldTransform` in C++
* port `FieldTransform`
* make `fieldTransforms` shared inside `context`
* Implement Precondition in C++ w/o test yet
* add unit test for `Precondition`
* port `Precondition`
* address changes
* address changes
* fix bugs for integration test
* address changes
* fix lint
* address changes
* address changes
* implement Firestore DatabaseInfo in C++
* temporary stash changes; blocking on the massive renaming of .m to .mm
* add database_info_test to project
* finish port DatabaseId and fix style, modular fixing DatabaseInfo
* port DatabaseInfo
* remove FSTDatabase{ID,Info} and their tests from project
* fix unit test
* use namespace alias
* use namespace alias, leftover
* address more changes
* refactoring to use raw pointer instead of value for property
* address changes
* remove self->
* fix style
* remove the name suffix Alloc
* fix a bug
Gil