Implement Deref and DerefMut for ScopedProtocol

[ctrlaltmilk]

Since ScopedProtocol's only use is to be dereferenced, it makes sense to add a Deref impl so users don't have to do it manually. This also improves safety, since with this change it's harder to accidentally keep a reference to a protocol alive after exiting boot services.

[nicholasbishop]

Thanks for the PR! I like the change to add Deref and DerefMut, but I also want to think more about whether it's OK that the final call sites no longer have any unsafe. The current convention that accessing a protocol always requires the call site to use unsafe predates my time working on this project, so also want to hear what @GabrielMajeri thinks. It's possible that the specific issues the unsafe was guarding against are more of a problem with handle_protocol than with open_protocol, but I'm not sure.

[GabrielMajeri]

The current convention that accessing a protocol always requires the call site to use unsafe predates my time working on this project, so also want to hear what @GabrielMajeri thinks.

The logic was as follows: in the early stages of uefi-rs, we didn't want to mark as "safe" functions which could in fact be unsafe (but not explicitly marked as such due to our lack of foresight). So the safe approach was to mark the creation of protocols as "unsafe", as a disclaimer to indicate to the caller that it's their responsibility to check that their usage of the protocol doesn't lead to potential UB.

I think it's OK now to make these methods safe, and fix any unsafety bugs which we encounter along the way.

[ctrlaltmilk]

In my personal opinion, I think the constant use of unsafe blocks "devalues" the idea of unsafety when using this library. While I do agree that it's fundamentally important to mark unsafe behavior, I also think that if there's a core abstraction to the library designed to prevent unsafety, it should be as user-friendly as possible. To that end, I'm open to doing work on ScopedProtocol myself to make it safer - if there's a specific holdup to this, just let me know and I can start work on fixing it.

[GabrielMajeri]

@verticalegg I agree with you. Let's try merging and using Deref and DerefMut as is, and we'll take steps later to eliminate unsafe uses of it -- if it's the case.

[nicholasbishop]

Great! I think that's a very nice usability improvement :)