This is safer and better matches our design (and the other platforms), since
classes like api::DocumentReference will have references to api::Firsetore but
the lifetime is controlled by the developer, not us.
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.
* Replace FSTDocumentSet's guts with immutable::SortedSet.
* Port FSTDocumentSet to C++ DocumentSet.
* Replace implementation of FSTDocumentSet with C++ DocumentSet
* Update all usages of FSTDocumentSet
* Remove FSTDocumentSet
* Add Objective-C++ guard
* Use absl::c_equal instead of hand-rolling it.
* Split SortedContainer out of SortedMapBase
... and use it
* Fix NSNotFound -> npos
* Remove = default constructors that aren't true.
These are implicitly deleted because DocumentSet is a member and isn't
default constructible.
* Fix usage of FSTQuery.limit
Gil