Add wrappers for SecItemDelete and SecItemUpdate

Thank you for the great library!

This PR adds `delete_item`, `update_item` and `ItemUpdateOptions`. The
functions reuse `SearchItemParams` to identify which items to modify
and so I've also added a `to_dictionary` method to `SearchItemParams`.
In fact, it would probably make sense to replace
`SearchItemParams::search` with a `search_item` function in line with
the add/update/delete API, but I didn't want to make unnecessary
changes.

Also in the spirit of minimising changes to existing code:
- I added a slightly strange API to `ItemUpdateOptions::set_class` to
  allow for rewriting the secret data without overwriting the class.
  Alternatively, I could add an `ItemUpdateValue` that is the same as
  `ItemAddValue`, but doesn't take a class.
- `ItemUpdateOptions` is basically a duplicate of `ItemAddOptions`. I
  could instead define `struct ItemAddOptions(ItemUpdateOptions)` and
  then just forward all the methods.

Signed-off-by: Klim Tsoutsman <klim@tsoutsman.com>