WIP: add native rust-based initramfs generator, init and qemu wrapper

[ddiss]

Sangeetha and I have been working on some changes to replace Dracut with
a rust-based initramfs / cpio image generator. The main reasons for this
are:

• Some distros are moving away from using Dracut, so we can't assume it's
  locally available
• We only use a small portion of Dracut functionality: base and systemd
  modules, and kernel / user dependency gathering for cpio
• Dracut is slow: it forks many processes, stages all initramfs content
  and is mostly written in bash

The rewrite builds on my previous dracut-cpio implementation and adds:

• elf dependency gathering using https://github.com/cole14/rust-elf
• kernel module dependency gathering via native modules.dep (etc.)
  parsing (thanks @thackara !)
  • VMs still user regular kmod / modprobe.
• basic rapido-vm and rapido-init programs, to run qemu and start autorun
  scripts
• basic rapido.conf parsing

It's otherwise kept as minimal as possible, with rust-elf and the std
library the only major external dependencies. One single-file crosvm
argument parser is also bundled.

The preliminary benchmark results look good, particularly for initramfs
image generation (cut):

---------------+-------------------------------+-----------------------|
               | Before: rapido e4c6077        | After: rs_wip 9a90973 |
               | dracut-059+suse.769.g693ea004 |  rustc 1.91.0         |
---------------+-------------------------------+-----------------------|
simple-example |   2.389s +- 0.117             | 0.075s +- 0.001       |
cut            |                               |                       |
---------------+-------------------------------+-----------------------|
simple-example |   7.2942 +- 0.065             | 4.746s +- 0.003       |
cut+boot+exit  |                               |                       |
---------------+-------------------------------+-----------------------|
simple-network |   2.572s +- 0.135             | 0.098s +- 0.000       |
cut            |                               |                       |
---------------+-------------------------------+-----------------------|
simple-network |   7.460s +- 0.126             | 4.926s +- 0.011       |
cut+boot+exit  |                               |                       |
---------------+-------------------------------+-----------------------|

To try these changes yourself, check out this branch and run:

cargo build --offline --release
./rapido cut simple-example

You may need to change your rapido.conf file a little if you
use env variables or shell callouts.

I'm flagging this as WIP, as there are still a few things to do:

• boot with systemd as init, instead of only rapido-init
• clean up interface before locking it in (parameter naming, etc.)
• think about distro packaging / path assumptions
  • at the moment, it assumes bins are in target/release/* and conf is
    in the working directory
• improve test coverage

I don't expect to convert remaining cut scripts before merge. Dracut and
rust based functionality should be able to live side by side, although
rapido.conf parsing is much less flexible in rust: no invocations,
currently no env var expansion, variables must be wrapped in {}.