Fixed soundness of as_bytes

Fixed soundness of ComponentBytes

On first glance you'll find a lot of unsafety but most of it is (now) benign.
The largest parts are casting a transparent, repr-C wrapper struct to native
arrays or slices and forwarding impls of bytemuck::{Pod, Zeroable}
accordingly. There's a derive crate for bytemuck that may be used instead.
There are not a lot of safety comments but not a lot of invariants used
either.

A longstanding soundness issue of interpreting a user input type T as
bytes... This goes back to 0.2 at least where we have the following trait:

pub trait ComponentBytes<T> {
  fn as_slice(&self) -> &[T];
  fn as_mut_slice(&mut self) -> &mut [T];

  // Provides: does this obvious transmutation cast of `as_slice` result.
  fn as_bytes(&self) -> &[u8] { ... }
}

This is so unsound, the user can even return an wrong slice of an arbitrary
and just let this trait do the dirty work of inspecting its bytes.

In a more recent version we're allowed to even write into that byte slice!
Let us create null references, invalid enums, whatever your heart desires.

it's mine!

it's mine!

my own

my own