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.
* naively remove FSTPath import and source/test files.
* port FieldPath, part I
* port FieldPath, part II
* port ResourcePath, part I
* port ResourcePath, part II
* the grand commit to fix build errors
* use testutil:: helper instead of those from FSTHelpers
* fix test and lint
* use c_str in errmsg directly
* fix
* fix
* make code clean
* fix integration test I missed
* fix to avoid naming collision in preprocessor
* address changes
* address changes
* address changes
* fix: fieldMask are actually shared with different context.
* address changes
* address changes
Gil