shred: remove usage of Vec::set_len()

[Arcterus]

This PR removes usage of Vec::set_len() (#1729). It also avoids allocating a new Vec every time BytesGenerator iterates, so it is most likely actually faster than before. Note that there is still quite a bit of room for improvement in terms of performance (e.g. copying entire patterns rather than pushing them to the Vec one byte at a time) and readability.

There is at least one more instance of Vec::set_len() that needs to be fixed (it's in od again). We may want to simply review our usage of unsafe in general, as I imagine some of the cruft that's built up over the years can be removed.

[Arcterus]

Also, I noticed that this utility has no tests. I'm not sure I'll bother adding them in this PR, so I'll open up an issue later.

[Arcterus]

You may not want to merge this yet. I'm thinking about just trying to remove the allocation entirely since the max size is 512 bytes (with the way the code is currently set up).

[Arcterus]

The latest commit removes all Vec allocation in favor of using a fixed-size array and simply returning a slice with the latest block of bytes. I also took the liberty to convert the generation for pattern-based passes to copy the entire pattern at once rather than push one byte at a time.

[Arcterus]

Btw @sylvestre lmk if you’d prefer I rebase this, seeing as my third commit basically wiped out the changes from the first.

[sylvestre]

Nah, we can use the squash feature of github :)