Don't use using statements in Objective-C headers ... (#2827)

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.

Author: wilhuff

Firestore/Example/Tests/API/FIRQuerySnapshotTests.mm

@@ -35,6 +35,10 @@
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 
 namespace util = firebase::firestore::util;
+using firebase::firestore::api::DocumentChange;
+using firebase::firestore::api::DocumentSnapshot;
+using firebase::firestore::api::Firestore;
+using firebase::firestore::api::SnapshotMetadata;
 using firebase::firestore::core::DocumentViewChange;
 using firebase::firestore::core::ViewSnapshot;
 using firebase::firestore::model::DocumentKeySet;

Firestore/Example/Tests/API/FSTAPIHelpers.mm

@@ -40,6 +40,7 @@
 
 namespace testutil = firebase::firestore::testutil;
 namespace util = firebase::firestore::util;
+using firebase::firestore::api::SnapshotMetadata;
 using firebase::firestore::core::DocumentViewChange;
 using firebase::firestore::core::ViewSnapshot;
 using firebase::firestore::model::DocumentKeySet;

Firestore/Example/Tests/Core/FSTEventManagerTests.mm

@@ -37,6 +37,7 @@
 
 using firebase::firestore::core::EventListener;
 using firebase::firestore::core::ListenOptions;
+using firebase::firestore::core::QueryListener;
 using firebase::firestore::core::ViewSnapshot;
 using firebase::firestore::model::DocumentKeySet;
 using firebase::firestore::model::DocumentSet;

Firestore/Example/Tests/Core/FSTQueryListenerTests.mm

@@ -45,6 +45,7 @@
 using firebase::firestore::core::DocumentViewChange;
 using firebase::firestore::core::EventListener;
 using firebase::firestore::core::ListenOptions;
+using firebase::firestore::core::QueryListener;
 using firebase::firestore::core::ViewSnapshot;
 using firebase::firestore::model::DocumentKeySet;
 using firebase::firestore::model::DocumentSet;

Firestore/Example/Tests/Integration/FSTDatastoreTests.mm

@@ -55,6 +55,7 @@
 using firebase::firestore::model::DatabaseId;
 using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::DocumentKeySet;
+using firebase::firestore::model::FieldValue;
 using firebase::firestore::model::Precondition;
 using firebase::firestore::model::OnlineState;
 using firebase::firestore::model::TargetId;

Firestore/Example/Tests/Model/FSTDocumentTests.mm

@@ -27,6 +27,7 @@
 
 namespace testutil = firebase::firestore::testutil;
 using firebase::firestore::model::DocumentKey;
+using firebase::firestore::model::FieldValue;
 using firebase::firestore::model::SnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN

Firestore/Example/Tests/Remote/FSTSerializerBetaTests.mm

@@ -65,6 +65,7 @@
 using firebase::firestore::model::DatabaseId;
 using firebase::firestore::model::FieldMask;
 using firebase::firestore::model::FieldTransform;
+using firebase::firestore::model::FieldValue;
 using firebase::firestore::model::Precondition;
 using firebase::firestore::model::SnapshotVersion;
 using firebase::firestore::remote::DocumentWatchChange;

Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.mm

@@ -61,6 +61,7 @@
 using firebase::firestore::auth::User;
 using firebase::firestore::core::DatabaseInfo;
 using firebase::firestore::core::ListenOptions;
+using firebase::firestore::core::QueryListener;
 using firebase::firestore::core::ViewSnapshot;
 using firebase::firestore::model::DatabaseId;
 using firebase::firestore::model::DocumentKey;

Firestore/Source/API/FIRCollectionReference+Internal.h

@@ -18,11 +18,13 @@
 
 #include "Firestore/core/src/firebase/firestore/model/resource_path.h"
 
+namespace model = firebase::firestore::model;
+
 NS_ASSUME_NONNULL_BEGIN
 
 /** Internal FIRCollectionReference API we don't want exposed in our public header files. */
 @interface FIRCollectionReference (Internal)
-+ (instancetype)referenceWithPath:(const firebase::firestore::model::ResourcePath &)path
++ (instancetype)referenceWithPath:(const model::ResourcePath &)path
                         firestore:(FIRFirestore *)firestore;
 @end
 

Firestore/Source/API/FIRDocumentChange+Internal.h

@@ -20,13 +20,14 @@
 
 #include "Firestore/core/src/firebase/firestore/api/document_change.h"
 
-using firebase::firestore::api::DocumentChange;
+namespace api = firebase::firestore::api;
 
 NS_ASSUME_NONNULL_BEGIN
 
 @interface FIRDocumentChange (/* Init */)
 
-- (instancetype)initWithDocumentChange:(DocumentChange &&)documentChange NS_DESIGNATED_INITIALIZER;
+- (instancetype)initWithDocumentChange:(api::DocumentChange &&)documentChange
+    NS_DESIGNATED_INITIALIZER;
 
 @end