Hexdump removal

This is a WIP on getting !23 (closed) rebased and hopefully merged.

Approach:

I decided to cherry pick the commit and apply it, as there have been some signature changes and it was incompatible. Instead of reworking the case scenarios in detect_compression like in the original MR, I opted for playing it safe, and instead tried to get as close as possible to the original hexdump implementation.

There are still some areas I'm investigating, e.g.: the original MR was failing some tests, at least locally, for some reason the bytes being scraped were in a different order then expected, that suggests some kind of endian-ness issue, but I'm still trying to track that down to the root cause.

There's also something unrelated to the MR that I'm investigating, but perhaps it won't be part of this MR and a separate issue altogether: It's about lsinitcpio.detect_filetype. I suspect the initial check for "no compression detection" won't always work. In fact, I'm not able to get the expected byte reading with either the old or the new code, testing with my actual initramfs.