Update quiver hashcode dependency (#88)

Development on the quiver_* subpackages has been discontinued in favour
of the main quiver package at https://github.com/google/quiver-dart. The
code itself is identical, but is more actively maintained.

Updated the dependency on the tuple package to 2.0.0, since it also
depends on quiver 3.0.0. Both packages has been migrated to null-safety
at those versions.

lib/models/documents/attribute.dart

@@ -1,4 +1,4 @@
-import 'package:quiver_hashcode/hashcode.dart';
+import 'package:quiver/core.dart';
 
 enum AttributeScope {
   INLINE, // refer to https://quilljs.com/docs/formats/#inline

lib/models/documents/style.dart

@@ -1,6 +1,6 @@
 import 'package:collection/collection.dart';
 import 'package:flutter_quill/models/documents/attribute.dart';
-import 'package:quiver_hashcode/hashcode.dart';
+import 'package:quiver/core.dart';
 
 /* Collection of style attributes */
 class Style {

lib/models/quill_delta.dart

@@ -7,7 +7,7 @@ library quill_delta;
 import 'dart:math' as math;
 
 import 'package:collection/collection.dart';
-import 'package:quiver_hashcode/hashcode.dart';
+import 'package:quiver/core.dart';
 
 const _attributeEquality = DeepCollectionEquality();
 const _valueEquality = DeepCollectionEquality();

pubspec.lock

@@ -276,19 +276,12 @@ packages:
     source: hosted
     version: "4.1.0"
   quiver:
-    dependency: transitive
-    description:
-      name: quiver
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.1.5"
-  quiver_hashcode:
     dependency: "direct main"
     description:
-      name: quiver_hashcode
+      name: quiver
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.0"
+    version: "3.0.0"
   sky_engine:
     dependency: transitive
     description: flutter
@@ -300,7 +293,7 @@ packages:
       name: source_span
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.0"
+    version: "1.8.1"
   stack_trace:
     dependency: transitive
     description:
@@ -349,7 +342,7 @@ packages:
       name: tuple
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.0.3"
+    version: "2.0.0"
   typed_data:
     dependency: transitive
     description:

pubspec.yaml

@@ -12,9 +12,9 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
-  quiver_hashcode: ^2.0.0
+  quiver: ^3.0.0
   collection: ^1.15.0
-  tuple: ^1.0.3
+  tuple: ^2.0.0
   url_launcher: ^6.0.0
   flutter_colorpicker: ^0.3.5
   image_picker: ^0.7.2