Reapply the old `GetSlice` as `OptionSlice`, because sometimes it's needed to use `Option<usize>` parameters

Created by: stratact

This is the return of the old GetSlice, now known as OptionSlice, because when times your just returning an Option<usize>, say from using .get() or .find() method slices, you should have the work done for you and done correctly without you having to think about it. So here are the following changes:

• Adds OptionSlice which takes Option<usize> as parameters
• Moves both GetSlice and OptionSlice to a slice module
  • And publicly exports them there
  • Fix the imports for other files
• Use OptionSlice in the more appropriate places where GetSlice is not well suited.