* 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