Add hashcode and equals to value

This simplifies testing, as now one can compare like this:
```dart
expect(Value(1), Value(1));
```
Or a realistic example:
```dart
final capturedArgument = verify(fooDao.insert(captureAny)).captured.first.createdAt;
expect(capturedArgument, Value(DateTime(0)));
```

A test is still missing which would look something like this:
```
test('values support hash and equals', () {
  const first = Value(0);
  final equalToFirst = Value(0);
  const different = Values.absent());

  expect(first.hashCode, equalToFirst.hashCode);
  expect(first, equals(equalToFirst));

  expect(first, isNot(equals(different)));
  expect(first, equals(first));
});
```
I'm not sure where the test is supposed to be.

moor/lib/src/runtime/data_class.dart

@@ -125,6 +125,17 @@ class Value<T> {
 
   @override
   String toString() => present ? 'Value($value)' : 'Value.absent()';
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is Value &&
+          runtimeType == other.runtimeType &&
+          present == other.present &&
+          value == other.value;
+
+  @override
+  int get hashCode => present.hashCode ^ value.hashCode;
 }
 
 /// Serializer responsible for mapping atomic types from and to json.