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.
* Use python executable directly
* python2 is not guaranteed to exist
* scripts aren't directly executable on Windows
* Add Firestore v1 protos
* Point cmake at Firestore v1 protos
* Regenerate protobuf-related sources
* Make local protos refer to v1 protos
* fixup! Regenerate protobuf-related sources
* Remove v1beta1 protos
* s/v1beta1/v1/g in source.
* s/v1beta1/v1/ in the Xcode project
* Remove stale bug comments.
This was fixed by adding an explicit FieldPath API rather than exposing
escaping to the end user.
Gil