firebase-ios-sdk

Port user data handling to C++ (#1823)

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