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/assignees.go

@@ -155,7 +155,7 @@ func MakeIDsFromAPIAssigneesToAdd(ctx context.Context, oneAssignee string, multi
 	var requestAssignees []string
 
 	// Keeping the old assigning method for compatibility reasons
-	if oneAssignee != "" && !util.IsStringInSlice(oneAssignee, multipleAssignees) {
+	if oneAssignee != "" && !util.SliceContainsString(multipleAssignees, oneAssignee) {
 		requestAssignees = append(requestAssignees, oneAssignee)
 	}