* Manual Xcode project fixups
* Remove `firebase/firestore` from paths in Firestore/core
This renames all Firestore files under `Firestore/core` to remove the
interior `firebase/firestore` directory structure. This is motivated by
build failures under Windows where the filenames of intermediates were
too long.
Concretely:
* `Firebase/core/src/firebase/firestore` becomes `Firebase/core/src`
* `Firebase/core/test/firebase/firestore` becomes
`Firebase/core/test/unit`
Moving the tests into a `unit` subdirectory aligns Firestore with the
directory structure in
[AddNewPod](https://github.com/firebase/firebase-ios-sdk/blob/master/AddNewPod.md),
modified for C++ (see discussion in #3224).
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.
Gil