Enable access to hidden RAII guard components

[JarlEvanson]

Enables access to hidden (but not sensitive) components of RAII guards.

Exposing the old Tpl value inside TplGuard allows for retrieving the current Tpl safely as shown below.

// SAFETY: // // Tpl::HIGH_LEVEL is the highest possible `Tpl` level according to the UEFI // specification and so will be greater than or equal to all valid `Tpl` values. // Furthermore, we are only in `Tpl::HIGH_LEVEL` for the smallest possible amount of time. unsafe { raise_tpl(Tpl::HIGH_LEVEL).old_tpl() }

Exposing the OpenProtocolParams inside ScopedProtocol allows UEFI drivers to use uefi and not have to store duplicate handles in their private data structures, since they can access the one inside of the ScopedProtocol.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

I think this looks fine, but could you add a little background to the PR description -- what's the use case where you would find these methods helpful?

[nicholasbishop]

Thanks for the update, lgtm