refactor: use libgit free() functions instead of calloc()

lib/src/bindings/config.dart

@@ -284,3 +284,6 @@ void deleteMultivar(
   calloc.free(name);
   calloc.free(regexpC);
 }
+
+/// Free the configuration and its associated memory and files.
+void free(Pointer<git_config> cfg) => libgit2.git_config_free(cfg);

lib/src/bindings/reference.dart

@@ -215,3 +215,6 @@ bool isValidName(String name) {
   calloc.free(refname);
   return result == 1 ? true : false;
 }
+
+/// Free the given reference.
+void free(Pointer<git_reference> ref) => libgit2.git_reference_free(ref);

lib/src/bindings/repository.dart

@@ -446,3 +446,6 @@ Pointer<Pointer<git_object>> revParseSingle(
 
   return out;
 }
+
+/// Free a previously allocated repository.
+void free(Pointer<git_repository> repo) => libgit2.git_repository_free(repo);

lib/src/config.dart

@@ -17,7 +17,7 @@ class Config {
   /// [path] should point to single on-disk file; it's expected to be a native
   /// Git config file following the default Git config syntax (see man git-config).
   ///
-  /// [Config] object should be closed with [close] function to release allocated memory.
+  /// [Config] object should be closed with [free] function to release allocated memory.
   Config.open({this.path}) {
     libgit2.git_libgit2_init();
 
@@ -145,8 +145,8 @@ class Config {
   }
 
   /// Releases memory allocated for config object.
-  void close() {
-    calloc.free(_configPointer);
+  void free() {
+    bindings.free(_configPointer);
     libgit2.git_libgit2_shutdown();
   }
 }

lib/src/reference.dart

@@ -138,7 +138,7 @@ class Reference {
 
   /// Releases memory allocated for reference object.
   void free() {
-    calloc.free(_refPointer);
+    bindings.free(_refPointer);
     libgit2.git_libgit2_shutdown();
   }
 }

lib/src/repository.dart

@@ -14,7 +14,7 @@ class Repository {
   /// or to the working directory. For a bare repository, [path] should directly
   /// point to the repository folder.
   ///
-  /// [Repository] object should be close with [close] function to release allocated memory.
+  /// [Repository] object should be close with [free] function to release allocated memory.
   ///
   /// Throws a [LibGit2Error] if error occured.
   Repository.open(String path) {
@@ -237,8 +237,8 @@ class Repository {
   String get workdir => bindings.workdir(_repoPointer);
 
   /// Releases memory allocated for repository object.
-  void close() {
-    calloc.free(_repoPointer);
+  void free() {
+    bindings.free(_repoPointer);
     libgit2.git_libgit2_shutdown();
   }