LoadFileProtocol and LoadFile2Protocol

[phip1611]

Supersedes #945. Implements LoadFile and LoadFile2, including an integration test. The way we are implementing the LoadFile(2) protocol is roughly what certain Linux loaders do so that Linux can find its initrd. See code for references.

Steps to Undraft

• merge uefi: add BootPolicy type #1326 and rebase onto

Checklist

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

[phip1611]

@nicholasbishop I noticed that we don't really have a streamlined strategy or code base regarding what is alloc and not alloc. Sometimes we feature-gate functionality between alloc, and sometimes we provide "non-alloc` compatible functions where a buffer is specified.

IMHO, we could also "turn the switch" to only provide first-class citizen support for the alloc feature as I think that UEFI applications without alloc are very rare. I can't really think of use-cases.

The only people don't want to use alloc are the people in safety-critical domains where they need deterministic memory management. But I think those people won't use UEFI in the first place 😁

I think we should also tell a few sentences about our strategy on that in the documentation of the crate. @nicholasbishop

[nicholasbishop]

I agree it's worth thinking more about the alloc strategy, and even if we decide to keep everything the same we should indeed document our approach more clearly. (And if we do decide changes are warranted I would want to hold off a bit as we already have large-scale changes happening in the repo.) Want to open an issue for this question?

[phip1611]

Want to open an issue for this question?

#1303

[phip1611]

Ready for the next review-round.