Adopt bytemuck crate

[cmyr]

This replaces our internal FromBytes trait with bytemuck::AnyBitPattern.

I'm not sure if we're ready to pull the trigger on this yet, but I wanted to at least get a patch ready and see if it presented any unexpected problems. It did not.

• This lets us remove the vast majority of unsafe from read-fonts. The only remaining unsafe here is trivial, and easily replaced with an unwrap:

  fontations/read-fonts/src/tables/post.rs

  Lines 75 to 78 in 247c7c3

  	if pstring.is_ascii() {
  	// SAFETY: `pstring` must be valid UTF-8, which is known to be the
  	// case since ASCII is a subset of UTF-8 and `is_ascii()` is true.
  	Ok(PString(unsafe { std::str::from_utf8_unchecked(pstring) }))

• By using the derive macro provided by bytemuck, we can be generally confident that we will not end up inadvertently generate unsound trait impls.
• This requires us to add two bits of unsafe impl trait to font-types, but these are localized and easy to reason about.
• overall I think that this is a good tradeoff, and I am happy to not be responsible for maintaining any fiddly unsafe here.
• see Consider using bytemuck as replacement for read_fonts::FromBytes #808

[drott]

Please see #808 (comment) - ideally let's hold landing this at least as a mandatory dependency until Chromium adds bytemuck.

[rsheeter]

Please rig for bytemuck 1.13.1

[dfrg]

🚢