* 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).
* Remove rarely used template argumnets from SortedMap/SortedSet
* Remove unused STATUS_CHECK_OK
* Rename QuerySnapshot::Listener to QuerySnapshotListener
This makes the type something that can be forward declared.
* Rename DocumentSnapshot::Listener to DocumentSnapshotListener
This makes the type something that can be forward declared.
* Rename ViewSnapshot::Listener to ViewSnapshotListener
This makes the type something that can be forward declared.
* Put most QueryListener implementation in .cc
* Actually use sync_engine_callback.h
* Add forward declaration headers for api, core, and model
These centralize forward declarations, including declarations for some
hard-to-forward-declare types.
* Move includes of hard_assert.h out of other headers
* Make testutil definitions out of line
* Move definitions out-of-line
* Move DocumentKey definitions out-of-line
* Move definitions out-of-line
* Shorten unnecessarily long namespace usages
* Use default destructors where possible
* Delete unused MutationBatch::kUnknown constant
* Use forward declarations instead of IWYU
* Misc fixups from Linux/GCC 9.2.1
* Fix testutil under VS 2017
Forward declarations of functions local to a function body end up
generating unresolved externals. Reduce most of these back to just
calling FieldValue::FromX methods.
Gil