firebase-ios-sdk

Use C++ serializer for communicating with the backend (#4013)

Replace all uses of `FSTSerializerBeta` in `remote_objc_bridge.{h,mm}` with the C++ `Serializer`. Also, make most of `remote/` sources pure C++ and remove no-longer-used methods in `FSTSerializerBeta`. All integration tests pass, under Address Sanitizer as well.

Unfortunately, pretty-printing is still delegated to Objective-C, so that `remote_objc_bridge.mm` still has to be an Objective-C++ source (and which prevents inlining `remote_objc_bridge` into the streams).

#no-changelog Will add a changelog entry when local serializer is ported, too.