use namespace aliases instead.
This makes the Objective-C code much more like what it's going to end up
being like when translated to C++: headers will include classes declared
in a namespace relative to firebase::firestore so everything else will
be named relative to firebase::firestore.
Meanwhile the using statements in the headers were causing us to miss
adding using statements in source files which was moving us away from
where we wanted to be.
Also eliminate most usages of `FSTDocumentKey`, remove most methods from the Objective-C class and make it just a wrapper over `DocumentKey`. The only usage that cannot be directly replaced by C++ `DocumentKey` is in `FSTFieldValue`.
This simplifies the import process back into google3 and allows us to add additional directories to the project without needing to update the project files for each directory we add. This shows up most clearly in the change to Firestore/Example/Firestore.xcodeproj/project.pbxproj: this no longer needs to list essentially every directory in the project as a header search path.
* Clang-format configuration for C++ files
* Add support C++-only sources to the podspec
* Podspec support for fully qualified imports
* xcodeproj changes for fully-qualified imports
* Use fully-qualified imports in Firestore sources
Gil