Drop unstable maybe_uninit_slice and vec_into_raw_parts features

[nicholasbishop]

The functionality we need from those features can be provided with a few pretty simple one-liner functions. Add those functions to the uefi crate in a new polyfill module.

#452

Checklist

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