add missing SecKeychainDelete function

[softprops]

Thanks for your work on this crate!

I'm looking into integrate it into a new project and found you can do most operations with keychains except for deleting them. It seems there is an api for deleting keychains but I couldn't find the interface in this crate that maps to it

[kornelski]

It would be good to have it.

Could you make a pull request for it?

It seems that Apple's function is "overloaded" and takes either array or an item. That doesn't fit Rust well. I suppose on the Rust side this should be expressed as two functions. Something like delete_item(&) and delete_items(&[])?