[BLOCKED] uefi-raw: unified boolean type

[phip1611]

This streamlines the existing usages of u8 and bool. Note that BootPolicy from #1297 is seamlessly integrated into u8 and bool.

Steps to Undraft

• close LoadFileProtocol and LoadFile2Protocol #1297 and rebase onto
• reevaluate the discussion from uefi: add BootPolicy type #1326 (comment)

Checklist

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

[nicholasbishop]

bool is an interesting case in terms of how far to go with worrying about undefined behavior. Creating a bool from any value other than 0 or 1 is UB. (Miri correctly catches it: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8777a1510372038e487aea18979f78ac.) The UEFI spec says pretty much the same thing: 0 and 1 are valid, other values are undefined. However, in C code it's not actually that hard to accidentally end up with a boolean-ish value that isn't 0 or 1. For example: bitwise operators like flags & MY_FLAG -- in Rust you have to explictly convert that to a bool with (flags & MY_FLAG) != 0, but not so in C.

So this ends up being similar to enums, where we carefully construct wrappers with newtype_enum! -- they can hold any bit pattern, unlike normal enums. If we want to guard against UB, then the Boolean type should be a repr(transparent) wrapper around u8, rather than a Rust bool. I think it's definitely questionable whether it's worth worrying about, but that's the explanation for why I used u8 rather than bool in most places in uefi-raw.