Implement FromStr / something that allows storing byte slices / yoke for KernelObject

[n0toose]

Currently, KernelObject could be used like this:

let elf = fs::read(&kernel_path)?;
let object =
	KernelObject::parse(&elf).map_err(...)?;

However, I asked around (after having worked on hermit-os/uhyve#725) as to how I could prevent the "use-after-free" warning that prevented me from storing the object object in the UhyveVm struct - and the problem seems to be that we depend on references that get "destroyed" soon afterwards:

hermit-entry/src/elf.rs

Lines 32 to 35 in 5c87198

	/// A parsed kernel object ready for loading.
	pub struct KernelObject<'a> {
	/// The raw bytes of the parsed ELF file.
	elf: &'a [u8],

A potential solution could be implementing something similar to the following trait: https://doc.rust-lang.org/std/str/trait.FromStr.html

[mkroening]

This is not yet possible without using raw pointer manually (which we should not do):

• Yoke vs noalias unicode-org/icu4x#2095
• What are the uniqueness guarantees of Box and Vec? rust-lang/unsafe-code-guidelines#326

We can keep this issue open for the future, but currently there is nothing we can do. I'd suggest keeping only the Vec around and reparsing it if necessary. The overhead should not be too bad.

In Uhyve, parsing ahead of time should also become unnecessary once C applications become relocatable.