Improve utils of slices (#22379)

- Move the file `compare.go` and `slice.go` to `slice.go`.
- Fix `ExistsInSlice`, it's buggy
  - It uses `sort.Search`, so it assumes that the input slice is sorted.
- It passes `func(i int) bool { return slice[i] == target })` to
`sort.Search`, that's incorrect, check the doc of `sort.Search`.
- Conbine `IsInt64InSlice(int64, []int64)` and `ExistsInSlice(string,
[]string)` to `SliceContains[T]([]T, T)`.
- Conbine `IsSliceInt64Eq([]int64, []int64)` and `IsEqualSlice([]string,
[]string)` to `SliceSortedEqual[T]([]T, T)`.
- Add `SliceEqual[T]([]T, T)` as a distinction from
`SliceSortedEqual[T]([]T, T)`.
- Redesign `RemoveIDFromList([]int64, int64) ([]int64, bool)` to
`SliceRemoveAll[T]([]T, T) []T`.
- Add `SliceContainsFunc[T]([]T, func(T) bool)` and
`SliceRemoveAllFunc[T]([]T, func(T) bool)` for general use.
- Add comments to explain why not `golang.org/x/exp/slices`.
- Add unit tests.

models/issues/issue.go

@@ -1529,7 +1529,7 @@ func IsUserParticipantsOfIssue(user *user_model.User, issue *Issue) bool {
 		log.Error(err.Error())
 		return false
 	}
-	return util.IsInt64InSlice(user.ID, userIDs)
+	return util.SliceContains(userIDs, user.ID)
 }
 
 // UpdateIssueMentions updates issue-user relations for mentioned users.
@@ -2023,7 +2023,7 @@ func (issue *Issue) GetParticipantIDsByIssue(ctx context.Context) ([]int64, erro
 		Find(&userIDs); err != nil {
 		return nil, fmt.Errorf("get poster IDs: %w", err)
 	}
-	if !util.IsInt64InSlice(issue.PosterID, userIDs) {
+	if !util.SliceContains(userIDs, issue.PosterID) {
 		return append(userIDs, issue.PosterID), nil
 	}
 	return userIDs, nil