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

1 files changed, 4 insertions, 5 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/document_key.h b/Firestore/core/src/firebase/firestore/model/document_key.h
index 8e590d8..134bda5 100644
--- a/Firestore/core/src/firebase/firestore/model/document_key.h
+++ b/Firestore/core/src/firebase/firestore/model/document_key.h
@@ -55,6 +55,10 @@ class DocumentKey {
   operator FSTDocumentKey*() const {
     return [FSTDocumentKey keyWithPath:path()];
   }
+
+  std::string ToString() const {
+    return path().CanonicalString();
+  }
 #endif
 
   /**
@@ -83,11 +87,6 @@ class DocumentKey {
     return path_ ? *path_ : Empty().path();
   }
 
-#if defined(__OBJC__)
-  // Helper function to convert to FSTDocumentKey during the C++ migration.
-  FSTDocumentKey* ToFSTDocumentKey() const;
-#endif  // defined(__OBJC__)
-
  private:
   // This is an optimization to make passing DocumentKey around cheaper (it's
   // copied often).