Define the function `delete_prefix(prefix, s)` which returns the string slice `s` with the `prefix` removed wrapped in `Some`. If `prefix ` is not contained in`s` it returns `None`.
Define the function `delete_prefix(prefix, s)` which returns the string slice `s` with the `prefix` removed wrapped in `Some`. If `prefix ` is not a prefix of`s` it returns `None`.