port DocumentKey to non-container types of Model/* (#930)

* 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

* add converter function between two DocumentKey implementations

* add unit test

* address changes

* fix lint

* using DocumentKey in model except for the container types `FSTDocumentDictionary`, `FSTDocumentKeySet`, and  `FSTDocumentVersionDictionary`

* change other place w.r.t. the use of `DocumentKey` in model

* update parameter of test helpers from NSString to string_view

* revert a temporary change used in debug

* address changes

Firestore/Example/Tests/API/FSTAPIHelpers.h

@@ -18,6 +18,8 @@
 
 #import "Firestore/Example/Tests/Util/FSTHelpers.h"
 
+#include "absl/strings/string_view.h"
+
 @class FIRCollectionReference;
 @class FIRDocumentReference;
 @class FIRDocumentSnapshot;
@@ -34,7 +36,7 @@ extern "C" {
 FIRFirestore *FSTTestFirestore();
 
 /** A convenience method for creating a doc snapshot for tests. */
-FIRDocumentSnapshot *FSTTestDocSnapshot(NSString *path,
+FIRDocumentSnapshot *FSTTestDocSnapshot(const absl::string_view path,
                                         FSTTestSnapshotVersion version,
                                         NSDictionary<NSString *, id> *_Nullable data,
                                         BOOL hasMutations,
@@ -61,7 +63,7 @@ FIRDocumentReference *FSTTestDocRef(const absl::string_view path);
  * @returns A query snapshot that consists of both sets of documents.
  */
 FIRQuerySnapshot *FSTTestQuerySnapshot(
-    NSString *path,
+    const absl::string_view path,
     NSDictionary<NSString *, NSDictionary<NSString *, id> *> *oldDocs,
     NSDictionary<NSString *, NSDictionary<NSString *, id> *> *docsToAdd,
     BOOL hasPendingWrites,